This is an interesting topic!
I would ideally like to purchase a car, learn what it is all about stock and then decide on what mods are required. Everyone has a different idea of what power or handling mods are needed or what constitutes 'light mods' and I feel it has just become an absolute nightmare reading opinions on forums or facebook or youtube as they are essentially big adverts alot of the time.
Handling mods are alot more personal and judging handling is a very personal thing
I have done a few little events with the 4C and gotten progressively better and more confident with it... running around the same times at one as a track-prepped supercharged Exige and at another I was a couple of seconds off some very serious things without really trying. This is the first mod that nobody does which is the driver mod...
I am still far from good, but the point is that NOW I can judge whether something needs to be changed or not outside of the basics. You have to do this from scratch for every car IMHO. In all this time I have owned the car, I have read about how you 'have to do' certain mods to the 4C immediately after taking delivery... the narrative was started when the car was new and owners took delivery of poorly aligned cars and before the revised suspension. My car has the race suspension which eliminated most of the issues you hear about online. Even then the issues were on the road and not the track... but still today you will find guys saying you have to immediately spend R6K on suspension blocks, R7K on upgraded ball joints, uniballs etc etc... Owners are doing this not even knowing what the car felt like stock or whether there was a real problem or not. They don't even know what these things are actually fixing. They have no idea if the car in normal form was a good match or not for their natural driving style... I could go on... There are guys afraid to track the car reading these things. When you learn yourself and the car you can hone in on what specific thing is required for YOUR specific use case or problem. It is easy to throw the kitchen sink at a car with all the available options. I had the same experience with my S2000... BMW community has the same things that crop up - especially with coilovers and springs.
For this reason I would not buy a car with extensive and very specific handling mods done... nor one that just has 'everything' done to it. Coilovers would have to be very specific brands which very few people in SA seem to have the appetite to invest in.
Power mods
I want to talk about power mods next. I don't mind someone having done software and downpipes on a car. What I do care about is what they have fitted where it was done and how it was used. This requires you to know and trust the person selling it. I would personally rather buy a car with power mods than one that has been returned to stock.
There is also such a thing as too much power... and even though a stock-internal motor will HANDLE a certain amount of power doesn't mean you should constantly be running that. I feel alot of guys are doing this because they are sponsored by motorplan or VISA and have a warped understanding of how this whole thing works... There is this mindset that because my mate with his N54/B58/S55 (who might be lying about his mods, mind you) is making a certain amount of power on allegedly pump fuel, I should be too or else I am missing out on something... The humble bragging (or lying) by tuners posting about their 10 second cars with 'safe daily files' and 'pump fuel' adds to this... Most of you who have spoken to me since doing Bootmod3 on the M5 will know that I think it has far too much power with the stage 2 map. It is a very aggressive tune and will even pull timing on high octane fuel in some scenarios. This is on a car that probably had too much power to begin with. I have no regrets going this route for other reasons, but it is running a far milder tune 99.9% of the time... because I am not a competitive racer or doing highway pulls and neither do I have sponsors or a long list of clients able to pay for any issues that arise.
Anyway, the point of this is there are cars running more aggressive maps than this (or setups with less control) all day... these same cars end up being sold after having a great deal of their useful life taken away because of it. You see these GT-Rs advertised with AMS Alpha kits for basically the price of the mods sometimes or Subaru's with 'freshly built' forged motors: Seeing the mods list of a car tells you absolutely nothing except that the owner had a big bank balance or large overdraft. You don't know how much life is remaining in those parts or what issues await you. That is when the tears start for the new owner when they think they are buying a car with under 50000km and wonder why it has problems that even 150000km cars don't experience. In the case of power mods, there is also another side to this: the buyer has to trust that the person is being truthful on one hand AND the seller has to trust that the new owner won't be a complete moron and knows what he is getting himself into... buyers remorse with modded cars gets very ugly.
For this reason, I am also hesitant to sell a car to somebody that I have modded (or a modded car in general) because I feel like my reputation is then at stake and in the hands of someone who perhaps has some other idea of what to expect (or treats it badly) and then I end up as that guy who sold someone a poorly modded car to them that broke and took them for a ride. It will be murphy's law that my otherwise well looked after car breaks something shortly after someone takes delivery and then I am painted as some villain. The car community is just that childish or vindictive alot of the time unfortunately and then everyone will get on the bandwagon to support the poor sod who bit off more than he could chew. Probably 80% of the reason I have kept my M5 is sentimental value and 20% is because of this.
I have numerous examples of people buying cars, maintaining them extremely poorly or with their 'trusted' junk-tier mechanics that have never seen such cars before and then turning around and claiming 'shortcuts were taken by previous owner' or 'previous owner was an <insert derogatory name here>' or ripped them off somehow. Honestly I don't have time for the drama. I think of selling my M5 and immediately think about the person immediately using crap fuel, shortcut fixes, destroying gearbox, motor, axles etc etc etc and then coming here to create a thread to extort money out of me to fix it. THIS SCENARIO HAS HAPPENED TO PEOPLE. No thanks!
Cosmetics
Cosmetically it is a matter of taste. As long as it is not some rocketbunny/LBW kit or generic autostyle stuff glued onto it in some irreversible way its really fine. If anything there are certain cosmetic things I would be drawn to and would make it a more attractive purchase.
