Exactly what Shackeng says.
We have a source for some tried and tested chips - unlike many of the trash we've seen from ebay. These won't turn it into a sports car, but make it into a much more drivable machine, with better power delivery. Brings it back into the region of the original BMW spec, approx. 150bhp (GM detuned to 120bhp). It transformed my TD Elite. It also improved (when not thrashing it!) MPG by around 10%
The downside of chipping these is the fuel computer reads wrong - it will read about 15-20% over (ie, I used to get about 31mpg over a tank from my auto, the MID claimed I was getting 37ish)
If its an auto, you will need a stronger gearbox. The TD + Auto combination isn't anywhere near as economical as the TD + Manual combination.
The viscous isn't required. Noisy, power sapping, inconvenient, useless piece of junk. If you tow, I might consider it, otherwise, don't bother. Remove the plastic cowling around rad as well

The TD needs regular oil changes (every 3k).