From what i've been reading, turbocharging an RX8 can be a tricky business, but can be done reliably if the job was conservative with boost, used high quality parts, had a way to deal with the extra heat (heat is the enemy real bad for rotaries) and had a proper tune. If any of those things weren't done, then it's a ticking time bomb, but i guess the same could be said for any engine. Also I read that the by-the-book way to wire up the engine management on the Greddy kit makes the rotors cry, though I have no actual experience. The Renesis has a higher compression ratio and smaller apex seals than the 13B-REW (not as deeply set into the rotors), so maybe that's why it's a bit tricky. The ignition coils are also somewhat inadequate, and the fuel pumps tend to do the lean-under-heavy-cornering thing with a mostly empty tank.
Plus, the 13B-REW engines were meant to be turbocharged and they still blew up whenever the hell they felt like it- would you feel comfortable with a 13B-MSP that was never meant for forced induction, and yet was turbocharged by a mystery man who may or may not have beaten this car (you can bet the salesmen have, and any kid who came in for a test drive, too)? Also, did I read 6,900 miles right? Seems awefully low. Low enough that any longevity problems would still be hidden away, and this car may have been sitting for long periods of time. I got mine about a year ago and I already have 7,000!
Also, how big is your son? I've fit a 6'1" football player in the back seat of mine (though he was sitting behind my fiance, who is very short) and he didn't complain loudly enough for me to listen. My Fiance's mother even had enough room back there to fall asleep! I couldn't believe that because I have an R3 and riding in the back seat is like having an angry chiropractor pummeling your spine out of alignment with every bump...