Depends on whether it really does need a "full rebuild" - a full rebuild is £2k absolute minimum - but you said it was taken off the road due to head gasket failure? I'm hearing two different things here.

If it needs a full engine rebuild then Tabetha's price of £3k - 3.5 sounds right.

But I'd like to have a good look at that shell rust-wise after all that time sitting in a barn.