I'm still learning about my own 300TD, but here's part of my upgrade path. I live waaaaay in the mountains and my vehicle is a daily driver/farm hauler/camper.

15-inch wheels, HD springs and shock absorbers, and a stronger brake booster (in which case the safe load capacity of the station wagon rises from the standard 560 kilograms to 700 kilograms) (I don't know where that info is from, nor do I know any more details. yet.)

flatface hubcaps. 99% faster to clean than bundts.

Tow hitch

A real roof rack

Better alternator

Built in 12 volt air compressor

Built in 12 volt DC-->120 volt AC generator (rediline)

3rd row seatbelts (seat is in, no seatbelts yet)

off road lighting

redundant street legal lighting (parallel LED bulbs so the car is still legal if a bulb fails).

12 volt DC-->5 volt DC convertor to supply usb ports