RE: APS-C - With a booster you'll be getting a 1.1x to 1.13x crop, so a 24mm would be roughly 27mm with the same extra stop of light you'd be getting with your MFT option. In other words, wides are still wide. Is it close enough to full frame? That's for you to decide. The Rokinon's still an option here, where it will have a 16mm-17mm equivalent field of view. I own both a NEX-3N and an A3000 and am also contemplating getting a focal reducer. Both cameras can be had for less than the $200-300 you had in mind. In fact, you could potentially have an A3000 and a Zhongyi LT II for under $300:
- Purchase a refurbished kit from Amazon ($190 shipped), or buy used. I may have gotten lucky, but the refurb I bought a few weeks ago had a shutter count of 00002(!), so it was new or as good as new.
- Sell the SEL1855, which will be in like new condition, to bring the cost of the A3000 down to $100, give or take. Just wait for mine to sell before listing yours - don't oversaturate the market

- Purchase the LT II for roughly $150 (perhaps in EF since you want to stick to M42 and OM).
- Purchase simple M42 to EOS and OM to EOS adapter rings.
Deciding between a NEX-6/5R/3N/5T/A3000/A5000/A5100/A6000/A6300 + FR is a dilemma in and of it self. Here are some opinionated things to note about the NEX-3N and the A3000.
- Both are very light-weight and can adapt to just about any lens.
- To me, the grip of the A3000 is essential for hand holding adapted telephoto lenses or anything heavy. With smaller lenses, I've actually grown to prefer the grip (or lack thereof) on the 3N.
- As many have said before me, the EVF on the A3000 is bad, but that is not to say it isn't useful. Once you learn to trust it, you'll feel better about it. The hard eyecup doesn't work well with eyeglasses. Focus peaking and focus magnification alleviate some issues with the EVF. The fixed rear screen is also sub-par. The 3N's screen is better and good enough.
- I believe the 3N is the first Sony MILC to allow wired remote controls via the MTP port (there are extra pins hidden behind the normal USB ones). Models released after it also have this port. A third-party intervalometer can be had for peanuts.
- LENR cannot be turned off on the A3000. The 3N can turn it off.
- RAW is not in the A3000's menu - one needs to use RAW+JPG.
- The A3000 has the bulk of a small DSLR, so when you add the length of the lens adapter and the adapted lens, it's potentially less compact than a small DSLR setup.
- The A3000 apparently has a very thin filter stack.
- If I had to choose between the A3000 and the NEX-3N, I'd choose the 3N. If I could afford an A6300, I'd just get an A7 series and ignore video

RE: FF - Yes, an A7[X] Mk. [Y] would likely be the best solution. It would save you time, effort, agony over decisions, and is the most direct route to full frame legacy lens use. You don't have to worry about the optical quality of a focal reducer, you'll have focusing aids for which a plain 5D OVF has no substitute (aside from an aftermarket focusing screen), and you'll get the full field of view for any FF lens you can get your hands on.
Then you'll only have to worry about things like sensor reflections, ghosting, adapter flare, filter stack thickness, shutter shock, light leaks, etc. *devil face*