Don't sound like there's any massive issues here but you might need a bit of time to work though them.
Check out Clarks garage for great troubleshooting and "how to" guides
http://www.clarks-garage.com/Also download the Porsche parts catalogue - PET
https://www.porsche.com/uk/accessoriesa ... catalogue/The Haynes is a US manual - can't say i used it much as it was not much use for the later cars but might be fine for early models.
1. No internal fans at any speed. Relay under the dash or resistor I assume? Fuses are fine.
Usually a resistor problem on the fans - under the scuttle
2. Should the center console clocks have illumination?
3. Voltmeter and oil pressure gauge inoperative. Car has had a brand new oil sender.
4. MPG gauge inoperative.
May all be connected? Earths may be suspect? Take apart & get busy with your multimeter?
5. Low oil level light lights up. Could this be the oil switch? If so where is it located?
oil level sender in the sump (not sure if this is present on early cars) - a new one is v expensive. You can buy a blanking plate and just remove it.
oil pressure sender on front o/s of block
6. Where are all the earths located around the car?
Should be diagrams on the web. Certainly worth a good clean up.
7. Wiper motor on front is squeaky. Just a strip down and regrease should do the trick?
no idea, sorry
8. If i were to get a set of D90 alloys, what offset etc would I need to get them in?
They will be the wrong offset & will need massive spacers. All in all a bad idea (particularly when you could run Fuchs..)
9. There are no rear seatbelts - should there be?
Probably not, although you might have anchor points for lap belts