Hi, there.
You need to have a bit of a read on this forum for common problems, but don't panic, because all of the things that can cause issues are fairly easily fixed; then they're bulletproof.
First is the CCT's.
Unlike most bikes, even those (like the Blackbird) with the same CCT fitted, Storms can (if the CCT fails, the internal spring on the front one is the usual culprit) slacken the cam chain enough for it to jump two or more teeth and this means that valves hit pistons.
Easily remedied with fitting manual CCT's- not a big job but you can get lots of help and advice on here.
Secondly, that old Honda bugbear of the regulator/rectifier- again a standard solution is to fit one form an R1 or R6, again once done you can forget this forever.
The third thing you'll notice is the cough you get through the airbox when the carbs are out of balance. Again, easily rectified and people on here will help you out with balancing if you're not that confident.
That's about it, really, they're fairly understressed and so don't wear out. Chains, steering head bearings, tyres all need to be ok, but that's the same for any buke.
When looking for one I'd be looking for an owner who knows about the bike, and has ridden it fairly regularly.
Carbs (like any other make or model) can gum up if it's been standing for a year or more it'll need the carbs cleaning properly.
Popular mods are K+N or similar air filters, race cans, but if these are fitted I'd hope to see some evidence that it's had a Dyno Jet kit properly set up on a dyno at some stage.
Watch out for a bike with standard cans on, but a jet kit fitted- it means the original owner's stripped off the cans to go on E-Bay but hasn't put the jetting back to standard.
And if you get a bike with race cans you'll need standard ones for the MOT, although you can pick these up for £30-50.
That's it, really, don't be worried by bigger mileages if the bike's been looked after by someone who knows what they're doing, and keep an eye on the "sales and swaps" section on here.
Last bit of advice- this is a great forum, everyone helps each other out and there's none of the bitching and arguing that normally happens.
So you've come to the right place!
Good luck with your hunt, and it may well be that another owner will be able to come with you to cast an eye over a prospective purchase if you let us know what area you live in.
BTW, this is all relevant to the Firestorm; if you're considering an SP1 or SP2 then one of the owners is better placed to give advice, they're different- for instance they both have gear-driven cams, so can't have CCT failures etc.
