Hi Ravi,
Welcome to the forum. Are you looking at the older style (discontinued) model with a separate gearbox or the latest UCE model? On either type, generally the less restricted the intake and exhaust are, the more pronounced the "thump". On the older "iron barreled" bikes this involves fitting a high flow aftermarket air filter and removal of the "hot tube" from the exhaust header. Or simply purchasing and fitting an after market exhaust head pipe and silencer. You'll then almost certainly have to re-jet the carburetor to handle the enhanced flow. It's probably the single most popular performance mod, and if you're buying used it may have been done already. I'm pretty sure that there is a growing after market for the UCE bikes as well.
As for style, you won't find many unbiased opinions here.
Maintenance. Well, it's an old fashioned kind of motorcycle that needs some diligent looking after. A lot of it is the same shit you'd do with any bike. Tire pressure, fluid levels, grease and adjust the chain and cables. Keep your battery charged up, check around for loose bolts or fraying wires, change blown bulbs. On the older bikes with points, check the gap and adjust if necessary; clean'em off while your there. Five minutes once a month. Same with the valves. The pushrod adjusters live behind a small door that comes off with one bolt. Another five minutes, seven if you're anal. It's those last two, the points and the valves that scare off most people. No reason. Everything I just mentioned will become second nature in no time. Believe me. Last thing I'll say about maintenance is CHANGE YOUR OIL EVERY THOUSAND MILES!
The stock drum brakes on the older bikes are adequate for the speeds you can ride at. Look five cars ahead and leave extra following distance. Also, Bullets have a lot of engine braking. One of their sweetest qualities in my opinion. The newer bikes have a disk brake up front. An improvement I guess.
I'm not sure if by "gear" you mean clothing or transmission.
The stock lighting is kinda dim and dorky looking but you can change that easily.
The clutch is wet and cable actuated.
Good luck with your purchase.
Chuck.