If you’re after an investment, keeping it relatively factory is mandatory.
These cars fetch the best prices but you are restricted!
My experience with modifying the car over 20 years of ownership led to fitting Motorsport oriented parts and it toughened the car up and helped reliability.
As usual, with all older cars of euro heritage, servicing can add up if they have been neglected but will make them a much better drive. There are plenty of trick parts out there on the net for suspension that mean, if you’re going to need to do bushes etc, you can save to buy the trick parts and just overhaul the whole lot in one go.
Again, if you don’t mind modifying, a 75 TS box is a must - better ratios and LSD.