If you're considering modifying a camera to shoot infrared, or just want to try out IR photography without spending money to convert you camera to IR, 720 nm is a good middle-of-the-road IR conversion to get, or filter to try. It gives you a bit of contrast and you can get good B&W renderings.
A 720 nm filter is almost the only IR filter that can be used with unconverted digital cameras if you want to try IR photography without modifying your camera, but in bright sunshine you'll have long exposures on the order of 15 seconds, and must use a tripod. I bought the Hoya R72 filter and used it on my camera for a while before deciding to dedicate a camera to be converted to IR.
The longer cutoff filters (850 nm, 900 nm, 920 nm, 950 nm give you even more of a B&W high-contrast result than 720 nm, but all of them require some kind of IR-converted camera.
If you go the 720 nm camera conversion route, you can use the longer IR cutoff filters mentioned above as well, but the shorter cutoff filters like the 590 nm and 650 nm filters would have almost no effect since the 720 nm IR filter installed over the sensor for the 720 conversion already blocks the shorter wavelengths.
720 nm gives you just a little bit of 'IR color', if you bring up the saturation and vibrance of the image in post-processing. If you color swap the red and blue channels (a common infrared technique) you can get a bit of blue in the sky and the foliage will show a pale yellow.
it's worth considering a full-spectrum camera conversion, which gives you the largest world of creative visible light, IR, and even UV photography opportunities. With full spectrum you'd be shooting with a lens filter on the camera for most of the creative options. Some folks always use a filter for lens protection anyway.
if you shoot night photography or astro, you can use a full-spectrum camera without any filters, which lets in about twice as much (or 1 EV more) light as an unconverted camera. it almost steps you up to the light-gathering equivalent of a full-frame, unconverted camera.
A post in this thread (see link below) shows a full range of creative options for a full-spectrum camera (and the options for other conversions as allowed by the conversion filter's cutoff):
https://www.dpreview.com/forums/post/66344116
Kolari and LifePixel, two popular conversion services have great sections on their web sites explaining the various conversion options. I had my conversion done by Isaac Szabo, at $175 for the M200 he was $100 less than the standard conversion services, and he did a great job.
If you go the full-spectrum conversion route and only have the EF-M 15-45mm kit lens, Amazon has an unbelievable deal on the Tiffen Hot MIrror filter for the 49mm size only (if you're in the USA) - this is the size needed for the EF-M 15-45 kit lens... it's only $10.65.
I've thought that this might be a price mistake as most other versions of the filter around that size are over $50! The 77mm filter is also fire-sale priced (showing for me at $60.81 today - it fluctuates wildly in that size) -- I bought it a few weeks ago when it was less than $50! The next day it jumped to $175 which is the 'normal' price for that size.
https://www.amazon.com/Tiffen-49mm-Hot-Mirror-Filter/dp/B00004ZCL4?th=1
The Tiffen hot mirror filter turns your camera back into a visible light camera for normal photos. Its spectral curve is not exactly the same as the factory hot mirror filters removed from the camera during a full-spectrum conversion, but you can do a custom white balance either in camera or in post-processing (if shooting raw) and get a normal-image with vibrant green foliage instead of reddish-tinted foliage, and slightly more vibrant reds.
A good filter for color IR is the Hoya 25A Red filter, which cuts off around 600 nm and passes IR light very well. It's a high-quality, common filter that's less expensive than the dedicated Kolari, LIfePixel, or Spencer camera multi-coated 590 nm filters which are slightly more orange but give similar results. You can often find the 25A filter at good prices secondhand.
A Hoya or Tiffen deeper Red #29 filter also is great for IR, with a higher cutoff around 620 nm, but they're less common and more expensive than the 25A filter.
The best-known filter for IR is the Hoya R72 filter, with a 720 nm cutoff. It's a very high-quality filter you can often find secondhard for a good price, since they're fairly common.
I just bought a brand-new Neewer 950 nm ultra-contrast monochromatic IR filter in 67mm size for only $6 off ebay, which included shipping! I thought, at that price I didn't have much to lose, but it turns out to be a great filter with good sharpness and high contrast! 950 nm cutoff requires longer exposures than the other IR filters since it cuts off so much of the IR spectrum, but in bright sunshine or on a tripod it's still usable with a converted camera with shutter speeds of 1/30 second or so at higher ISOs like 400-800, and gives good results.
For me the full-spectrum camera is the 'safest' route to go. If it turns out that IR is not your cup of tea, you just use the hot mirror filter and you can use your camera as a normal visible-light camera again.
I often take the M200 along in my camera bag as the second or third body, with the EF-M 15-45mm kit lens and the 49mm Tiffen Hot mirror filter on it. I can swap and use that lens with that filter on it on my unconverted M6ii cameras, and it gives normal images on those cameras. Then if I want to shoot IR, I can remove the hot mirror filter and put on the Hoya Red 25A or R72 for color or B&W IR and shoot with the full-spectrum M200.
Generally filters in 49mm size are cheaper than other 'standard sizes' such as 55mm or 67mm so if you're sticking with just the kit lens (which is a very good infrared lens for the EF-M system) you can do it inexpensively. If you think you may get other lenses for the system, you'd be safer going with a larger filter size such as 55 mm or 67mm.
The most common filter size I've bought for IR is 67mm since I can use that size with all of my lenses (some with step-up rings). The Rokinon 12mm f2.8 and Sigma 16mm f1.4 lens use 67mm filters. I've also bought a 77mm to 67mm step-down ring so I can use all the 67mm filters on the Ef-mount Rokinon 135mm f2 lens for astrophotography. 67mm filters do not vignette on that lens.
I also have a few 55 mm filters like the Hoya Red 25A filter which I found for cheap on eBay in that size, and 55mm will work on all the EF-M lenses (some with step up rings). I have permanent 43 - 55mm step-up rings on my EF-M 22mm f2 lens and 32mm f1.4 lenses, and don't understand why Canon didn't make 55mm the filter size for ALL EF-M lenses as it works perfectly for the EF-M standard 'lens barrel' size. I have a 52-55mm step up ring for the EF-M 55-200 lens, but unfortunately this ring blocks the lens hood from being mounted on that lens.
I have several 55-67mm step-up rings so I can use the 67mm filters on the EF-M lenses as well, and a 58-67mm step up ring so I can use 67mm filters on the EF-S 55-250 IS STM lens. I bought lens caps off Amazon.com in packs of 4 in both 55mm size and 67mm size for less than $10 per pack. I sometimes leave the step-up rings on lenses for shooting sessions, swapping lenses and filters without fiddling as much with step-up rings.
The lens hood for the 32mm f1.4 fits nicely over the 55mm step up ring. I have a 49mm to 55mm step up ring I often use with the 15-45 kit lens, and a 52mm to 55mm step-up ring for the EF-M 55-200 lens. The Canon EF-M 18-150 and Sigma 56mm f1.4 lenses are already 55mm filter size.
