Don't buy stunt bikes used, you are waisting your money, those bikes unless are new & pricey, will most likely be F-up junk metal someone is looking to drop and get a grand or so out before that thing is useless.
Plus if you go that way you won't find your own style or comfort speed, or have the setup suited to you.
Best way to start is to buy an older model 600 in the 2500 to 3500 range, that has never been stunted before and is in mint shape. then take off all the fairings, so you have a naked bike, buy frame sliders at first or best crash bars side and rear, the rest like rear hand brake kits, sprokets all can be added slowly as you gain comfort and skill, you won't be needing a rear hand brake anyway till, or unless you can hold slow or fast wheelies pulling high chairs, jump 50/50's or rolling drifts in those positions and so on stuff like that.
as you get the basics of this new stunt bike of yours, you can start ordering parts online, for reasonable prices you can get kits from $400 to $900, if you can't afford to do all this and need a balls out used pos stunt bike with problems thats been grounded, you might not want to get into this sport, and you have to build your own bike, because you will need to spend hours fixing it after every session, in over 20 years i've never had to send any of my bikes to a mechanic did all the work myself, this way you know your isht, and i started as a noob too, didn't go to any mechanic school or anything i just did the hard time!
if you are just getting into it because its cool, stay away, you'll just hurt yourself. with all the things cleared if you are serious, get ready to crash ALOT, fix alot and have alot of fun! lol