There arent to many common problems really, clicky rear axle which is a couple of hundred quid to fix. Skipping bose stero can be sorted.
I did my clutch just over 30k. Just fitted a lighweight clutch and flywheel and its a lot better.
I think as a general rule you look at the value of the base car and then selling the mods you will get back about 50% of the original cost.
Its always a difficult call with modded cars. If its what you want there are not many lkike it around but the market is very limited for the seller.