First time I replied I missed the fact that the car had been modified. Too bad!
The SLS system would have been better off left in the car. I hope the springs are the properly for ride height and quality and you don't ever haul lots of weight in the rear because the springs can't possibly compensate like a SLS would and the headlights will be up in the air if you put a load in the rear.

I've been cross country on several trips in my Turbo converted '79 and my turbo '85 wagons
They are hard to beat for loading lots of stuff easily. The '79 is an NA (non turbo) engine and not the fastest thing on the highway, you might want to find a later wagon with a turbo in it if you plan on lots of driving at much over 60 - 65 MPH because the gearing in the differential in the '79 is pretty low and the engine will be wound up pretty fast at those speeds.
You asked how much to convert back to SLS, well, the shocks are about 700 set of two, about $110 for two accumulators, a used SLS valve might be $25 or it might be $200 depending on where you look. Hopefully the pump is still on the car but if it had no oil in it the impeller is probably bad and a new one is $1100 (or more if you buy from the dealer). Fluid reservoir (better find a used one) New springs $200. bits and pieces another $$$$
I think you better find another wagon if you want a working SLS.

DDH

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!