No idea what the underside will be like if the top has so much surface rust. Good ones can be had for £35k ish, so doesn't leave a lot to restore this at £24k.
MOH 35 is Charles Lumbys car it is truly stunning and has been for years, £24k for a car that needs quite a lot of work seems pricey as it will cost more than what it is worth to get it finished.
Get it on wheeler dealers. Ed china will buff that off and put in back on the road in an hour for £500
If you have a look at their other items for sale there is a whole bunch of cars listed for the same auctions including a Mk1 Golf GTI, a QP and an M3 along with some nice looking classics including a stag. Guide prices seem to be all over the place.