You pose some interesting questions, and I'm sure you will get numerous replies.
There are some great bargains to be had in Japan, but shipping costs and arrangements can be prohibitive for one-offs and low volume. I reckon you will be better off buying a bike already imported into the EU, and maybe like you say, the UK. I believe transfering the registration between EU countries isn't too difficult, and that for a while you can even use a foreign registration (possibly 6~12 months?).
In my opinion the MC21 is the best model to go for. Many parts are still readily available, and support is exceptional for a 15 year old 2-stroke! They are also easily delimted and derestricted. The MC28 is a great looking bike, and many prefer the trick parts like the digital display and Pro-Arm, but the initial purchase price is around £1000 (at least!) higher the the equivallent MC21 model, and delimiting and drestriction is both more expensive and less productive!
The MC18's are a cracking bike, but parts are getting scarcer all the time, and many new parts are discontinued. The MC18 is starting to look a little dated now too, but that can add to it's charm if that's what you are after. Power-wise, there is nothing between the MC18 and MC21 for the best part. In full race trim the MC21 will win out, but for 99% of the population, you'll be hard pressed to tell the difference.
A number of the owners here have owned RGV's, and most wouldn't ever go back! I'm one of them! As an officially supported UK model, RGV's (and KR1S) were hard to beat, but as a motorcycle the NSR tramps it! The build quality is far superior, and the motor is in a whole different league. Not until the RGV VJ23 did Suzuki up their game to the build-quality level of Honda (and Yamaha), maybe even surpassing it, but by then it was too late.
Although most of the 250's produce similar power once delimited/derestricted (read the GT Performance Proddy 250 Leader Board thread), the NSR has the greatest potential utilising stock parts, and the smoothest power delivery. This makes some of the other makes "feel" deceptively quicker due to their steep power curves.
For the majority though, it comes down to personal preference regarding the looks. I think the MC21, 3XV and VJ23 all look like they mean business, but look into the cost and time involved to make them "go", and there is no contest! Try getting a reliable 65hp out of the 3XV and VJ23...
You can do it with the NSR with a soldering iron, a hole cutter, and some careful port-matching, and all with standard engine parts - 60+hp with just the soldering iron and a hole cutter!! 
If you go ahead and buy an NSR, I will be interested to hear what you think of it compared to your RGV's once you've delimited it! Even in stadard form though, I think you will be impressed! It seems that the RGV die-hards dismiss all other marques without ever even trying them, but those that have ventured from the Suzuki path seldom go back!!
Hopefully our F250 Racer will prove itself next season, depending on how fast our rider turns out to be! It should be one of the more reliable bikes out there though, I am confident of that!
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Andy.
NSR-WORLD.COM
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