There should be enough bits in existing cars to get you on the road but some sticking points that come to mind are:

1) SLS = IIRC all SECs have rear SLS which means you'll need an SLS pump or convert the SEC to coil springs and shocks. The easiest way to keep SLS is to ditch the engine you have on deck and get one with an SLS pump in the head such as from an 81-85 wagon. You might be able to attach a tandem (PS and SLS) pump but I haven't heard of anyone doing that in a 617. I mean it's such an easy solution to using a non-SLS 617 in a wagon that I'd expect to have read about it.

2) Prop shaft = you might need a custom length prop shaft.

3) Fuel tank = you might need to customize some fittings if the sedan tank doesn't fit the SEC.

4) Transmission = use a transmission from a W126, not a W116 or W123, so you have the proper speedo drive. If you swap gauge clusters, use a speedo calibrated for the diff.

Sixto
87 300D