I bought a stock 00 Z28 at 76k, she's at 94k now and absolutely NO motor problems, trannies and rears are always gonna be the way they are, they'll blow whenever they feel like it. But as far as the ls1 goes, ur not going to get one that has never seen a burnout or been raced, unless you buy a single owner 02 w/ 2k on it and the owner was an 80yr old lady...and even then...lol. But these motors, when not under the control of a douche, are billetproof (lol lil play on mustangs) If you tune it up, keep it maintained, and be suspicious of every little thing, she'll last as long as you can keep up. I personally know 3 guys with 300+k on stock LS1 C5 Vettes. Heh it's a shame i know, but they drive those cars EVERYWHERE and just simple things like water pumps, timing chains, keeping an eye on plugs/wires can make these things last like a beat honda. Only much faster and happier. GL man!!
ps if your going to work on it, go for a 00+. The 98-99s are a F-in pain to work on.