For me it depends on the car. I've seen some really nicely modded classics: a Vitesse with a 2.0L Zetec and 5 speed box, a gold Scimtar SE6 with Granada front and rear bumpers and Jag XK seats, a lovely Sebring replica MGB with uprated engine and suspension (replica Minilites) and so on. I'd happily buy any of them. If I could afford one I'd buy a Z-Cars Mini tomorrow just to play with on Sundays!
However; with current (ish) models, an something I am going to drive daily, modifications always put me off. I'd accept a remap (if it was done by a good name with all documentation) and an DPF/EGR delete (again - documentation), possibly some subtle cosmetic mods (upgraded interior, ICE, wheels etc) - but nothing aftermarket, and I'd walk away if lowered suspension, cat back exhausts, bigger turbos etc were mentioned.
Reading this back I suppose if the mods were something I'd do (if I had the time and money) I'd accept them (but look very carefully at how well they'd been done) but if they went beyond that I'd be very uncomfortable. I like to a few things to my cars but prefer to keep it subtle. I've had a remap (why not!) and DPF delete (no brainer), fitted Parrot Blue Tooth (too many RNSs getting stolen) got a nice set of mats, and have upgraded to a Sachs clutch when mine started slipping (but that's another story - see my post:
http://www.audi-sport.net/vb/a3-s3-sportback-8p-chassis/164781-new-sachs-clutch-slipping.html ). I will probably install Cruise Control (once I have the clutch sorted), might get some better (Audi) alloys, and possibly a MFSteering Wheel. Might get the roof resprayed (bird poo damage which wont polish out). I've already got black leather, dual climate and the stereo is pretty good. Not bothered about elec/heated seats (I know they are nice) and not a fan of tints, spoilers, splitters and general 'bling'.
I really like having the power and handling there to use (when it's safe) but in as stock looking package. That's why I loved my (stock) S4. Spotty 'youfs' in their slammed Clios ans Corsas couldn't believe how quick the balding, pot bellied old git in his
"A4" was!