Hi Mick! and welcome.
Everyone has an opinion as to the best NSR. Its up to personal choice.
The 16 is more old school, and getting parts can be a problem. Getting more power out of a 16 is a lot of work also.
The 18 is regarded as the best engine for power , better brakes, suspension etc.
The 21, well everyones got one. Can make good power with very little work. lots of part avalable, body kits etc, some real trick ones about.
The 28. the most trick, and some would say the best looking. More work needed to get good power that the 21 or 18. If you can find one big money.
As for price, a good 16 can be had from £500. A good 18 £700. 21 £1200, and the 28 £2800+.
Have a look on thr forum at the NSR-WORLD two stroke track day at Llandow.