Well you do have a project on your hands. it can all be worked out with time and money.
I don`t know if you have any Pick N Pull type yards near you, but they can be a real source for good parts. especially since Obama decided a lot of good cars needed to be junked. there are a lot of nice W126 in these yards for parts. they don`t last long before they are crushed.
for the gaskets, I would give Phill a call in the buy parts tab above. yuou will get quality and the right parts.
as for the back glass with the delaminate problem , again PNP. think one is around $45. pull the "C" piller panels off to expose the defrost connections on each side. then cut the rubber from the inside and push it out with trim and all.
the sun roof is 3 pieces. two side felts, one on the rear of the lid. have to remove lid to replace it. and the one across the front. this one is a bugger to replace. but dooable, been their. lot less hair now
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fuel gauge could be the sending unit. need to pull it out and clean the gunk out of it. there is a DIY pictorial for doing it. might be for a 123 but is the same.
oil gauge might be the electric sender on the bottom of the oil filter housing, or maybe the wire came off.
temp gauge might be the temp sensor on the left side of the engine between the 2nd and 3rd injector I think.
Might be a good idea to pull the fuses and clean all the connections and clean off the ends of the fuses, or just replace them all with the copper ones. the aluminium ones seem to corrode.
this will be a good project for you son to learn on, and a safe car for him to drive. there is plenty of help here on the forum. get him on line here, and the experts will walk him through things.
Charlie