The problem is that it isn't only "honest rust" its years of English MoT patches badly welded in place. It really IS do-able but will take time and patience. Buying one of the cheap Chinese shrinker/stretchers off EBay for about £100 would be a good investment, they aren't the best quality but I've used mine time after time. A good set of doors (or repairing the existing ones to the right shape) is the first step I.M.O and making a template of the rear screen before repairing its surround.
It will be nice to see it restored to its original English condition.