We have two D1's; a 3 door and a 5 door. the 3 door is much nicer to drive, and handles much better, due the reduction in weight by having 2 less doors. (IMHO
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However, the 3 doors do rust more than the 5's - especially in front of the rear wheel arch, and it is much harder to get at and fix, so be careful. ( you can't take the door off ( obviously), so you really need to take the side panel off to get proper access. IMHO, any other way is a bodge - and If you are paying garage rates for this type of work, then you will need very deep pockets, and quite long arms too )
IMHO, unless you can weld, and weld well, I wouldn't go near one, unless its a re-import from Japan (etc), where there is no road salt. However, for the low mileage bit, you will have to pay towards £7K for one with about 40K miles on it.
Definitely lift the carpets and look at footwells and boot floor. If the seller isn't keen on this, walk away. definitely take a mate who knows what he is doing, and take a magnet too - flipping underseal can hide all sorts of rubbish pretending to be steel !
If you buy one, almost your first job should be to strip the underseal of it - only my opinion, but its shared by a few others on here - I think we have all learned the hard way