Rust. Remember that even the youngest Lightweight is almost 30 years old. Doors and footwells etc are buttons on ebay, chassis can be welded - but bulkhead vent panels are an arm & leg.
What engine is in it ? Has it been messed about with and what quality was the job ? Mine has a galv chassis - great - but a home-made loom - very not great at all.
12v or 24v ? If the later then you've potentially got some very heavy duty wiring but expensive spares if it goes Kaput.
Good luck - I've not stopped smiling since I bought mine.
G.