one of the biggest issues with the roadster is the glass in the top falling out. inspect carefully and also the hyd pump and rams that operate the top should be inspected. seat heaters break with regularity and self dimming mirrors also tend to fail.
the driveline on the n/a cars are just about bullit proof. you really wont have many issues there. trans sensor can leak as do the valve covers but minor fixes at best
if you have the oppurtunity i highly recommend you drive the srt-6 before you buy. the power is amazing and a much rarer bird than the limited, especially in roadster version