That MU is overpriced. Don't worry about the back end, they ALL sag... the rear leafs are not that great in them, but aftermarket replacements are available... just factor in $400 or so for new springs and bushes when you buy one, and possibly another $200 or so for new shocks in the rear.
Oh, and the rattle and squeak could just be the rear door and glass... the glass door can be quietened by spraying silicone spray on the rubber seal (also has the side effect of helping keep the rain out

) plus also the shagged rear suspension won't help.
Avoid the 2.4TD surf if you can't afford to fix it. You would be better off with the MU for reliability... the 2.8TD motor in the MU will go forever if properly maintained, and has WAAAY more power than the Surf will ever dream of. The cambelts are easy, but those motors do tend to start going bang around 180k onwards... which will be much harder to fix
I would expect to pay about $2500-3500 for that MU based on what you described.