A good 400cc bandit costs in between 120,000 to 200,000rs (again depending on model year and condition) Preference can be attributed to a number of things. Some prefer a bandit while some prefer crotch rockets (FZR, ZXR, GSXR etc) and then some prefer vintage straight bikes. Its a matter of personal preference in the end. Crotch rockets are relatively pricier than the rest. The newer the model year, the expensive it turns out to be.
Overall condition of the bike needs to be thoroughly inspected prior making a purchase, which includes engine, chassis and cosmetics/body. A sport bike mechanic would be your best bet. Like cars, a mechanic can be taken along for a thorough inspection.
A validly registered bike is whats legal, any bike with a number plate on it cannot be deemed legal. To ensure the bike you're going to be purchasing is legally registered, you may either call cplc or pay them a visit with the registration, chassis and engine number for verification.
For information, I believe the forum you are on is the best place for any details you may be on the lookout for. As for recommending a shop, I personally cannot vouch for any. May be someone else could enlighten you on that very end.
Hope the above information helped.