Congrats on going to the dark side. Nice looking 90, and already has Wolf wheels. I have had 4 jeeps and 2 DiscoI, 1 DiscoII, and now have 1 LR4 and 4 Defenders! It is addictive.
My 2 cents, while your dad is under the hood:
Ensure throttle cable adjusted properly
Ensure clutch pedal adjusted properly
Adjust valves to spec
Dial guage adjustment and timing of injector pump
Check turbo for excessive shaft play
Change fuel filter, put a few hundred miles on it, and change again.
Read a lot before you mess with IP settings ensure that you have it close to baseline/factory settings. Just my advise. Doug at Dividing Creek has a nice archive writeup, IIRC.
200’s are known for vibration. Either go with Glencoyne motor/drivetrain mounts, or with Bearmach. Any others are too stiff, this is based on a lot of people’s experience on DefenderSource and UK forums.
DynaMat and factory sound insulation/tunnel matting are a big help.
Don’t know what is going on with dash, but it appears to block the windscreen vents. Seat retrim kits are cheap, Exmoor really good brand, BritPart are good. Buy these from a UK vendor, easy to do.
Take a look at AdamSanta85’s 90 thread on Defendersource. Really good refresh of a 90.
Have fun!