Try get a Jasper that's modded. That way you can be fairly sure it hasn't been RROD before.
Here's how I identify a jasper:
Where it says 12.1 means it's a jasper, 14.2 means Falcon, 16.5 means Zephyr and no HDMI means Xenon (14.2 with no HDMI means Opus).
If it's a good deal (like including HDD, or controllers), the oldest model I'd go with is a Falcon and even still, you'd wanna do some work to keep it cool (fresh thermal paste, speed up fans, heatsinks on RAM + Southbridge + HANA) and be wary of RROD on them too.