Raj, i'm going to suggest something different to the others because i can't see them willingly refunding just like that and i reckon you can get what you want out of them.
A bit of negotiation needs to be done here, you've already put money into the vehicle so it might be worth letting the seller fix the vehicle for you.
I'd do away with the hydraulic suspension and suggest they fit a proper make (Pedders?) traditional suspension kit, you want new quality shockers all round and new brake and fuel pipes, plus new brake calipers because the current ones are already riddled with rust.
You also require them to get a professional bodyshop to systematically remove and treat the existing rust with rusted parts treated and painted properly, then a final coat of a preparation like Krown, including inside the chassis and box sections and all the body cavities too, all this at their expense with photos taken of every stage, you certainly do not want just a coat of bloody waxoyl spraying on...oh with a coating of waxoyl type stuff it'll look the dogs bollocks for about 12 months but underneath that rust will be creeping continually.
All this needs to be in writing of course so there's no misunderstandings.
They might not want to do this, but it's going to cost them more to refund you and then try to resell the car because its now currently a known problem.
Apart from the pipes calipers and shockers the chassis itself appears solid, so i believe you can still have a very good vehicle out of this.
Certainly i'd speak to them and see what they intend to do before it gets to bad mouthing bad publicity stage, course if they'll refund you that's fine too but what about the money you've already spent.
My opinion of how its got this bad is i suspect the vehicle ended up standing most of last year's mass house arrest after being subject to massive salt covering over the winter, possibly Scotland, unless a suggested it was a boat launcher. or the previous owner(s) liked driving on a beach.