I bought my Bandit 400 last year with around 48,000km on it and put around 6000km on it last year. It was my first bike, and I, like you, didnt know much about working on bikes at all.
I did all the usual stuff...changed the oil and filter, changed the brake fluid, did a valve check (which will require you to remove the radiator, so you'll be changing the coolant in the process), new brake pads and new tires.
She ran pretty flawlessly the entire season...except a few times where she would not start. I blame it on dirty carbs.
Basically, the bikes dont seem to have any major common problems that I can think of....if it starts and runs well, doesnt have a plume of smoke coming out the exhaust and doesnt sound like its gonna fall apart, then it should be good to go!
Oh..one issue does seem to come up on these bikes, which is a broken 2nd gear. Apparently, the B4 has a weak 2nd gear. Although, it didnt happen to me, nor have I seen anyone post about it recently, so I dont think its a huge issue. But I'd give it a test ride to make sure its shifting well.
Good luck! I sold my B4 this spring and kinda miss the little bugger :(