With a few added thoughts.....
....if you buy the car as an investment (or vertually any car for that matter), you'll be lucky to make money.
They are to enjoy, but i fully appreciate your concerns in the matter, as asked.
My own ethos is to enjoy the car as it is (or should be), spend money as and when it NEEDS it to keep it safe.....
...only then consider what you WANT, and spend according to your budget.
That said, certain expenditure can be said to improve value, whilst others only improve saleability - the trick is to know which is which, and some things appeal to some buyers more than others.
For example, when i bought the blue car, it was nice to know that it had new galvanised rear wishbones/top links, and a recent S2 hood - as far as i was concerned, it was 'worth' a grand to have them, even though they cost a heck of a lot more than that.
It was nice to have a FSH (but not essential to me!), a reasonable stereo, Cat 1 alarm, and a partially replaced interior (seat covers re-built) - but as far as i was concerned, they didn't affect value.
It had new tyres, but they wern't what i wanted, so although worth a mention in the advert (can't blame the seller for my fussiness!), they were actually a retrograde step (still got them if anyone wants tyres with loads of tread, but not one of the generally accepted brands for our cars)....
I have since spent loads on the car, but whether it's improved the value is debatable (i believe some has), but it certainly has improved the 'saleability', but more importantly, it has made the use of the car much nicer - i can get in and know that it will start, run and keep going (safely) to wherever i want to go - and be a better place to sit than when i got it.
These changes (i prefer that to 'uprades') include roll-hoops, Piper exhaust, Samco hoses, wastegate porting, re-built turbo, Switcher chip, radiator, oil cooler, coupled with regular maintenance of the important bits - brakes, oil, other fluids, belts etc.......
....all are needed to keep the car as I want it, may be what the next owner wants, but (probably/possibly/IMHO) do/don't increase the value, but definately make it stand out from the next one for sale at the same price!!!!