Real tough question. A lot of the "Tuner" parts are very high quality, but people don't try to beef up a performance car with the idea that they are going to not use the increased performance afterwards! If you want max performance and the car is otherwise clean with great records, go for it. If you want it to last and maybe even appreciate in value as a unique car, I would look for one that hasn't been modified.
Some like to wring the max out of a car and figure it was meant to be driven, others enjoy a rare automobile and baby it, never approaching the limits for which it was designed. I don't think there is a "right" answer on this one, more personal philosophy!
2003 Firemist Red/grey leather SL 500
2015 Palladium Silver/black mbtex GLK 350
1987 Smoke Silver/burgundy mbtex 300E Sportline (SOLD)
Click to see 87 300E