Series I Land Rovers generally increase in value according to their condition and the quality of restoration undertaken. They are increasingly becoming collectors items and an older model can be more expensive than a younger one ('younger' means less than half a century old! and non are younger than 48).
Land Rover bodywork is mostly aluminium but you need to check for corrosion where the aluminium is in contact with the steel chassis and bulkhead. The doors may be cracked, hinges worn and damage/corrosion around the striker plate. Supports for the hood are liable to rot, as are the rain channels. Check for repairs done with body filler. The steel bulkhead footwells can be fabricated and welded.
The chassis outriggers and crossmembers corrode but if you are looking for a restoration project they can be fabricated and welded.
All the electrics should function but remember that the dynamo fitted is not a powerful unit. Check the ammeter shows the battery charges. The wiper motors will probably be weak. Series II/IIa wiper motors fit. You could upgrade to an alternator if desired.
The steering wheel may be cracked and the steering box will be worn (can use SII or SIII unit but not for 80" models). Brake shoes can be used from SII and SIII upto the early 1980's.