I just looked at the beginning of your thread again, So this engine never ran, and you got it as an engine only? you took the head off and had it done , is that it?
what did the cyl bores look like with the head off? when it did run after the head, how did the lower end sound?
The differential compression tools are piston aircraft stuff. you can find them at the aircraft tool supply houses, there are a few, ATS, and aircraft spruce and specialty are two. BUT you will have to fabricate an adapter from either a injector or glow plug. you might also look for a leakdown tester, or, you might be able to do a cheapy check with come compressed air in your intake and exhaust. not sure how that would work on a 5 cyl. you just need to be at TDC on compression stroke for the cyl you want to check. but if the head was done you would hope it is good
now to things not to do to your engine, from what you have said so far I would not expect results from any of this, but you can pull injectors fill the cyl's and soak with a penetrant of your choice, ( marvel , diesel? , engine oil etc?) rotate engine reverse and forward and repeat for as long as you like. Then I would just blow all the penetrant out with injectors out and turn the starter. with the cylinders freshly oily, you can do another compression test. did you check your valve timing since you had the head off???

good luck, you are in pretty deep; to stop now. What do good running donor cars cost in your area???? Not driving, cars, but running

I pulled the head to do general cleaning and inspection, just figured that while it was off i would replace the guides and seals myself. The cylinders looked very good, as in I could still make out some cross hatching on the walls, very small ridge. After getting it back together, when I could get it to fire, it sounded like a nice idle, nothing clanking or loud knocks, just very short lived. If I do a wet compression test, and the compression goes up significantly, is that indicative of bad rings?

what about valve timing??????????????? Did you ever verify oil pressure? Did you starter fluid it, or ether it?? are you posting for some more specific MB advice on either peachparts or superturbodiesel.com. peach parts was pretty friendly
Heck pull the injectors, fill the jugs full of oil spin by hand a bit, let it sit overnight. Empty the primary fuel filter, and bowl, Fill the primary fuel filter bowl full of fresh SEAFOAM... crank a bit to blow out the oil from yesterday, do a compression test, put the injectors back in bleed the SEAFOAM thru and see if it will go...
if that does not make it run for more than 5 minutes on its own, I think you got some problems. I mean the MB is extremely hard to kill, unless someone just drove it into the ground which from your description of the internals is not the case.

from my opinion yes if your standard compression tester results go up after a fresh cylinder oil soaking then you could look at rings.

here is my 3rd gen temp tank/ fuel injector cleaner set up without the air regulator. you can use gravity or charge it with a compressor to whatever fuel pressure you want.

Any results? I hope you get your problem figured out!

Compression tests (wet cylinder) revealed faulty rings. this little booger was somehow beaten up bad. Weighing options, thinking pretty seriously about a rebuilt long block, and be done in one shot. Hmmmmm.