turbo's dont generally need rebuilt from sitting..so if seller is suggesting a freshen up,i would have it gone thru for sure..at 250. thats way cheap i think. As for 600hp..is that at the wheels or crank?...either way,always look first to be sure your basics are covered ie; scanmaster readings..basic tune-up items to start,coil pack output,go over all vacuum lines/hoses,as well as known good valve springs. Then fuel needs are an absolute must.. 60lb. injectors,at least,... a denso or bosch hot wired fuel pump,an adjustable f/press.regulator an adjustable waste gate for the t-60,a decent 3"downpipe, and exh.system,a new ecm chip and at this point,a rich girlfriend or wife is a big help! And then you need to read up on all the tuning info available here at this site ..then you may be ready for the best bang for the buck...alcohol injection..this will allow you to turn boost up past the 15-17 range (that you will be stuck at because of basic pump gas) or else it will knock and blow head gaskets without question...but the alky will let you run stupid amounts of boost and you car will be a blast and a beast....life starts at 25 lbs boost! and dont forget the tires...all told you may want to gather funds firsts..at least or 4 to 5g's sounds about right.