My 90 used to be a hardtop, bought a truck cab, made a bottom hinged rear door. Now on soft top, got rid of the TC, asked my local LR fixer and he reckoned a 90 hardtop with windows and rear door worth about 400 notes, add in some new seals and a few bits and bobs (you might need door hinges fitted and lock striker fittings) so prob 500 notes would leave you some beer tokens for the mate you'll want to hold roof/sides.
Used to take me 1/2 day to change over from full top to truck cab (on my own - why I have a bad back!)
Cheers
Gavin