Echoing what gts455 has said, I would just make sure you do your own investigating with the bike when you look at it. Any signs of it being laid down? Free spinning wheels? Chain properly lubed and tightened? The little stuff can usually point you in the right direction concerning the bike's life. Not sure why the PO powder coated the swing arm unless it's a different color. Was it rashed?
I'm not too concerned about the motor mods and other small things done as besides the P&P and pistons everything else seems pretty common amongst the RC51 convent. I would heed gts' advice and seek out or research the mech who did the mods, he might not remember doing it but his reputation will tell you if he's somewhat trustworthy.
Good deal, 5thou for a 12K sp2 with solid motor mods...sound like the perfect platform for beginning a new chapter of RC51 mania.
2001 #62. Ohlins rear/valves, Micron high mount, HyperPro damper, SharkSkinz, Tyga billet triple clamp, Tyga Carbon Fiber fender and hugger, Ti bolt/custom subframe/alien motion 8 cell diet, 15/41 520, COP conv. PCIII, Venhill lines, modified air box, turn signals in headlights...<deep breath> ZG Touring windscreen, Tech G rear sets, CRF450 rear master w/HRC cover, Tokico 6 pots w/V-pads, CRG Bar end mirrors, PSR Shorty levers-use only clutch, wanna-be basic racer paint, Roman Gold PC wheels