First of all, Do the ARB bushes really need replacing? They are coming up for the right age but it is equally likely that it is the droplinks that are knocking. Also, knocking ARB bushes aren't necessarily an MOT failure. If you can move the droplinks by hand with little effort then they are almost guaranteed to be knocking. Upper wishbones can also knock on rough roads. Both are cheap and trivialy easy to replace.
The ARB itself is a pain in the ass to replace but with good strategy it can be done. I have worked out a way to leave almost the whole suspension intact to replace it.
I would buy it and spend nothing fixing it straight away unless it is overdue a timing belt change but do treat the rust. Get a feel for it and what is wrong and what it is likely to need. Book it in for it's MOT 1 month before expiry then see what it fails on. you may get lucky.
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