I've written quite a bit about issues with import cars but a few days ago.
http://type-r-owners.co.uk/forums/showthread.php?68443-Help-thinking-of-buying-an-Integra-Type-R-DC5
The tax band depends on when the car was imported and who imported it. I would guess if you're going for the cheaper end of the market, you'll probably get a car registered as PLG which is the best tax band you could hope for.
The K20a engine has a higher compression ration than the K20a2, so arguable it will be slightly more thirsty, however having owned both variations, I can confirm that there's nothing in it and it all basically boils down to how heavy your right root is.
Parts can be a PITA to get if there are none in stock in the UK. After that it's a 2-3 day wait for them to come over from their main depot in Belgium, or if you're really unlucky an eight week wait from Japan. The parts aren't cheap either (a JDM EP3 will share many with the Euro EP3 though). As an example, a front bumper for an EP3 supplied painted from Honda is around £250. You can double that for a DC5 bumper. Don't even ask how much a C Pack rear windscreen costs either
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Servicing costs should be the same. Some dealers will do them on a S2000 schedule though for some reason?