Hi and welcome to the forum. The M sport seats do suffer from bolster wear, regardless of mileage. If you are looking at black seats, then the repair is easy. Just buy a dye from a supplier like Dynamix for circa £15, lightly sand the worn area with wet & dry, clean with white sprit, allow to dry, then apply the dye - very easy and cheap. If another colour, then the success of this approach will depend on the colour match with the dye. Worst case, you will need a pro to visit and spray in the bolster leather, only circa £50, so not a disaster.
Other things to check / consider;
- have rear springs been replaced ? (known early life failure issue)
- have front wishbones been replaced ? (again known early life wear issue)
- check fog lights and headlights for moisture (fog light seals can be an issue and headlight covers are difficult to get back on correctly when bulb are replaced)
- ensure electric windows run smoothly (regulators can be an issue)
- if fitted with run flats, ensure car tracks straight (worn run flats can lead to tram lining)
- ensure air in and heated rear window work correctly (flat batteries can cause the car to go into transport mode - no issue to clear but will need a visit to dealer / specialist to reset the mode in the ECU)
I recently bought a 2006 2.5si roadster and it is great. Very well made (albeit with the usual rattles that plague roadsters) and mechanically, issues above aside, the Z4 is very strong.
Hope this helps.
Happy shopping, James