Some interesting questions!
Front Suspension.
XT600 suspension is notorious, it is like riding a jelly. For the front people either:
Do a complete front end swap or mod the forks. Look in the 'workshop' section, I posted some links to lists of bikes with the same bearings/fork diameters etc for potential donors. Popular choices are kx500, yz, wr, kdx.
Fork iniminators from Ricor, cartridge emulators form race tech (or the cheaper pd valve from yss suspension) change the handling of the suspension.
New springs. hyperpro, wilbur, hagon, etc make progressive springs for the XT600 at around £80-£120 for a pair.
Rear Suspension.
The choices are really limited on this one. Hagon, Wilbur, Ohlin make replacement rear shocks. These range from £300-£650.
You can take a rear shock from a TT600 and bodge the frame. (coincidentally I have a works performance shock for a TT600 I am trying to flog

) If you look in the restorations thread there is one called 1991 army XT by danjet, it show the process.
Another alternative is simply up-rating the spring by 10-15% (no more in order to avoid re-valving etc).
Hyperpro make a complete spring set for the XT600e, this plus emulators for the front may be quite good?
Compatibility.
The early XT600 were mostly part compatible with the early Teneres however the XT600e was different from almost everything else. However there is a LOT more aftermarket stuff for the XT600e.
Jacked up!
The rear end can be jacked up with a metal spacer, there is talk of it on ADVrider or somewhere like that. It was a very simple metal piece that could be fabricated at home with a file and a vice.