I wonder how the 'mechanic' arrived at the broke rod diagnosis?
I look at it this way; a broke rod is definitely in FAVOR of the buyer!! You have all the leverage.
If this was a true 'hydraulic' issue;
Before I'd suspect the injector 'filling' the chamber, I'd figure a blown head gasket/cracked liner/cracked head allowing coolant to seep into that cylinder.. and the next attempt to start the engine resulted in a bent rod.
You'll see this on engines that tend to sit around. (if they in fact have a bad gasket, head, liner)
I've seen it many times; the owner gets tired of filling the engine with coolant, knowing there may be a problem.. Sits in the back yard for 6 months. Decides to sell it, and charges up the battery; Cranks it and <insert your choice of noises>
Piston comes up, and either
1. stops the starter dead
2. 'just' makes it past tdc, but stresses the snot out of the rod. Runs for about 10 seconds, then 'boink'. clatter, clatter, screech.
See if you can rotate a full 360.... 720 would be better. If you can roll it without any tightness, tight spots, definite clunks.. and you cannot see any holes, or other forms of escape attempts of rotating components.. You probably have at least a 'rebuildable' engine.
So, what I have seen in the past 4 months...
Buy a 'good runner' Liberty for $5000-6000, that has ALL the upgrades, (some of the 'upgrades' are now 6-10 yrs old, with 100k+ miles JUST on them.. And the body and the rest of the driveline has 200K+ miles!
Or... buy a 'project' (such as yours) for say, $1500-2000, (you have the upper hand negotiating with the 'broke rod') and go balls out on a total out of frame OH.
Worst case scenario; you have $4000 in the engine, with everything basically NEW. Now you have a 6000 investment with a ZERO mile engine. And you have a body and driveline with about 120k miles..
Don't even consider a salvage yard engine. Those $2500-3500 'good runners'... are a total crap shoot. If you can't hear it run.. just walk away. I've changed my thinking on 'take outs' over the years; they are only good for the re-seller of running vehicles. They buy a non runner vehicle, throw in a 'take out'... and peddle it ASAP. I WOULDN'T let my Mother in law drive it.. let alone my family. And that is at least 2500 you could've spent on BRAND NEW PARTS on a rebuild!
If you do have to find another building block to start with, you should be able to find a rebuildable core (short block) for $300-500 Lots of these sitting in salvage yards with the head gone..
Now, I realize that everyone can't do this, or has the tools, time, space to do this. This vehicle/engine probably isn't for them in the first place. This is why new vehicles roll off the assembly line every day.. But for the person who can invest themselves, then I say 'go for it'.