Hi there
Welcome to the forum! No FAQ guide that I know off (other more experienced members may advise differently) but this is SUCH a broad question that it is hard to give comprehensive advise... but I'm sure you'll get loads on here shortly!

Here's some of my thoughts:
You do have to be sure of what you want before you start looking at bikes - in particular are you after originality? For some people (and I have no problem with that at all) their bike has to be exactly as it left the factory. If that is you, then you will need to do plenty of research to find out what models were fitted with what, and when. If you don't care about that - do no research on this!
Legal stuff - check paperwork carefully. Are the engine and frame numbers as stated on the registration documents? Also, NEVER buy a bike from a guy who will only give you a mobile number and doesn't invite you in the house. Not so much with Classic Bikes maybe, but it is not unknown for crooks to meet up outside a house they don't own, to sell a car that is stolen, with only a mobile number as a contact!
Costs - do the tyres, chain and sprockets etc look worn out? You can be looking at a couple of hundred quid to sort those out, so negotiate well. Old cracked tyres aren't much fun to ride on!
Don't buy from a dealer - they general charge about 30% too much!
Good luck with the purchase
Adrian
