If that cabin has air in it other than what came from the factory, it's a hard pass for me.
But in all seriousness, that's not far off from where I'd be if buying second hand. A large number of people seem to like modifying their cars, but the vast majority also want to buy a stock one. It's a bit ironic, but that's how it seems to go.
For me, I think I'd look at modifications in a few camps:
Exterior Aesthetic Modifications
I tend to like simplicity in design, so I'm not the guy to add lips, wings, body kits, extra carbon bits, etc. I can appreciate some of these (when done tastefully), but it's not for me. Even simple things, like wheels or even badges, I just like them stock. Part of this, for me, is that aesthetically modified cars feel/appear more "used" to me for some reason.
Mechanical Modifications
If (a) the modification was done to solve a known mechanical issue, (b) it was done in a way that's widely accepted by the community, and (c) it was carried out by a trusted party (e.g., mechanic), I'd tend to look at these modifications as beneficial and valuable. For example, someone upgrading a coolant tank on the R8 so it doesn't run the risk of exploding? That's probably a good thing.
Now, if the mechanical modification is purely for the pleasure of the owner - especially in the camp of "performance upgrades" (TT, supercharger, ECU tunes, etc.) - I think these are red flags. Whether true or not, it's going to send signals that the car was driven hard and therefore potentially more "worn."
Electronics Modifications
I know many people just want the latest and greatest technology, but stuff like after-market head units and swapped speakers are a turn-off, too. Not only do the vast majority of these units not fit-in with the interior design, but more than a few people do poor jobs installing them - cutting wires, creating phantom power drains on the battery, etc. I'll gladly deal with old, stock technology that looks the part but doesn't give me the latest tech... after all, every few years it'll be the same problem.
In short, if I were a secondary market buyer, nearly all modifications, short of those done for mechanical sorting, would be a turn-off. A high quality exhaust, or high quality suspension upgrade (if not seeking mag-ride, let's say), would probably be a few exceptions to the rule here.