The 80-200mm f/2.8 would compliment your 18-70 well... giving you a kit with excellent image quality, but it will be a heavy and bulky kit because of the 80-200's size and weight.
The 80-200 was one of the first lenses I purchased after getting my D70 with the 18-70mm kit lens. I've never regretted that purchase, but honestly it isn't my most-used lens. I do use it quite often, and it has captured some of my best photographs. The image quality always reminds me why I lug it around. Along with my primes, it defines excellence in image quality as far as I'm concerned.
But... I don't take it everywhere. It's not a "let me throw this into my bag in case I need it" kind of lens. I do take it hiking when I hope to photograph wildlife, or distant landscapes (along with a Kenko 1.4x TC). I also love it for portraits and events. This lens is excellent for portraits, and it has been a bread & butter lens for wedding photographers and photo-journalists for many years. I like taking portraits and event photos with this lens primarily for the beautifully smooth bokeh. Of course it is also a great lens for action shots, but not the quickest to focus.
I have a 24-120mm VR that is my lgihtweight replacement. I hope to replace the 24-120 with the 18-200mm VR very soon (I'm on a waitlist). A superzoom won't give you the shutter speed, sharpness, or bokeh of the 80-200mm f/2.8, but it can still give you fantastic photos. In some situations a superzoom will get you shots that aren't possible with the 80-200 (wide angle shots and slow shutter handheld shots if you have a VR superzoom). Here are the times when I prefer a super-zoom:
When I want to be less conspicuous. There are times when I don't want to have a big, professional looking lens that attracts attention.
When I want to travel light: This is the big problem with the 80-200. It isn't much fun to travel around new places, shopping, sight-seeing, etc. with a large unwieldy bag of photo gear. There are times when I travel with the 80-200. If I know I will use that range a lot, and want the best IQ possible, it comes with me. If I'm not sure I'll use it, the big lens stays home. I've traveled quite happily with only my Tokina 12-24, 24-120VR, 35mm f/2, and fisheye lenses. For ultralight travel, I think I'd be happy with only the 18-200mm VR, but I'd supplement it with a fast prime like the 35mm f/2. I also have a hard time leaving the 12-24 behind. I'm a sucker for wide angles.
As an amateur, I don't want to take a vacation with my fiancee and make the trip all about my hobby. Sure I want to bring home some great photographs (and she wants me to as well), but I also want to enjoy myself and focus on spending time with my fiancee. I enjoy having a convenient lens that allows me to open my bag, zoom to just the right focal length, and take the picture. When I'm using the 24-120, I try to keep the aperture between f/8 and f/11 and it produces wonderful images. They won't always compare well against the same photo taken with the 80-200 or one of my primes, but quite often they do. Once and awhile I look at a photo taken with the 24-120 and wish I would have had a better lens, but most of the time I'm perfectly pleased with the results... and the lighter load!
Size and weight are relative, and the 80-200 is not that much of a beast compared to wildlife lenses. Many professional photographers and photo-journalists regularly carry a couple of pro-DSLR bodies and two or three heavy zooms like the 17-55mm f/2.8 and 70-200mm f/2.8 VR. If your style demands fast shutter speeds in low light, the best image quality, rugged build quality, etc... you've got to find a way to shoulder the burden (or hire an assistant).
For amateur photographers like myself, we have to weigh these factors against cost, convenience, and our own personal limitations when it comes to size and weight. For me personally... the 80-200 is more essential than the superzoom. But if I travelled more often, the superzoom might be in the bag more often.
Also kindly suggest a good portrait lens
or is my 18-70 kit lens good enough for this purpose ?
Thanks for yr time
As I mentioned before, the 80-200 is an excellent portrait lens... but only if you've got some distance between you and your subject. The 85mm primes are THE lengends as far as portraits go, but on a DSLR the 50mm primes are excellent as well. You can also get a 60mm f/2.8 macro that will serve as an excellent portrait lens. I often use my Tamron 90mm f/2.8 macro for portraits, but prefer the 80-200 when I have it with me. Your 18-70mm can also take excellent portraits, though it won't blur the background as well as a faster lens.
For candids and portraits that capture more of the environment, the 35mm f/2 is ideal in my opinion. I often use the 35 as a walk-around lens. Other primes usually make me long for a zoom if I'm walking around with them attached, but the 35 feels right in many situations.
You can never own enough lenses since each one has strengths and weaknesses that make them more or less suitable to different shooting styles and different situations. The trick is to decide which lens or lenses you will use the most for now, and start there. The good news, and the bad news, is that you can always buy more. You can sell them too, so don't sweat it if you buy a lens and find it just doesn't work for you.
Sean