Here's an overview based upon my own experience with this.........
I've added an aftermarket rear heater only but had flirted with and even bought a brand new factory rear heater-A/C unit for a whopping $125 shipped to my door. That was a once-in-a-lifetime acquisition and for a few reasons its now been sold.
Finding a used factory unit is somewhat easy but truthfully the heater core and A/C evaporator would be the first things replaced. Any potential mess a leaking pressure vessel leaves is bad enough in my cargo-only vans---if it was a built out conversion I can't imagine what chaos that would be. Count on over $250 just for those two parts alone. (We'll assume for now the blower motor, vacuum servo and resistor network are at least working at time of purchase.)
Next is changing the dash mounted fan speed and function control with the factory rear heater capable version. Not a costly part at all---$25 max through eBay, fairly easy install if you've already been inside your center dash board. There should be a loose pigtail lead connector that is wired to another connector in the left side B Pillar, near the seat belt lower mounting retracting reel.
At that connector you'll have signals from the dash mounted rear fan speed control, a 30 amp constant power feed and blue vacuum line, capped off until needed. The factory wiring harness for rear heat & A/C takes those signals, power and vacuum to the factory rear assembly. Typically if all these components are in place they're a simple plug-n-play thing---all except for coolant and refrigerant lines of course.
(Compressors are indeed the same----go figure but in the end that's a good thing cost wise!)
IF you're still with me now this is where the expense really kicks in. You'll have to run engine coolant lines from the van's front to the rear unit. Properly insulated and secured along their length is vital for several reasons. (My arrangement has an electro-vacuum control circuit of my creation allowing me to basically completely close off and isolate my rear heater when its not needed; Ford's arrangement has hot coolant flowing through the rear heater core always.)
Fittings for the A/C up front are another costly issue. If using the factory system you'd have to buy their fittings, no idea what they cost or even what they look like. From there suitable A/C lines connecting the condenser to the rear unit is as vital as the coolant lines. Equally necessary to insulate and secure these too. An advantage of sorts is these lines can be made by most hydraulic hose/fitting shops from hose that will withstand far more severe duty than A/C use. I would budget somewhere near $160 minimum just for these two lines with installed fittings both ends.
Once all components are fitted, installed and secured time to fill the cores--coolant is easy enough. A/C would require a qualified shop or person with necessary equipment to evacuate, charge and leak test the new parts.
And that's all there is to it!
^^^Mostly a tongue-in-cheek comment but if you're really intent on rear heat and A/C there might be conversion shops in your area willing to tackle this for you---I could see costs approaching $2500 or more. Since you're so far running a cargo-only van this is a bit easier but still fairly labor intensive. Without proper insulation this sort of refit doesn't make a lot of sense IMHO.
Glad to answer any questions you might have now--hope this was just a bit informative.