My experience with this engine:
I have an D53T engine that came out of a Dodge 50 pickup sitting in my shop. I am going to put it in a Ford Bronco 2 that I use to push my snow plow around with. (Ford used this engine in the Ranger for a few years)
It is disassembled right now, there is one broken piston ring in cyl #3 that left a small groove in the cyl. I don't know what caused it to break.
I will have to install a sleeve in it but I can still use the original piston with a new set of rings.
I did a valve job on the cyl head, but the camshaft and rocker arms will have to be reground, the lobes are worn. The later engines all have roller tips, so this is a weak spot on this engine.
My engine had 80.000 miles on it so I don't think that the previous owner had the oil changed regulary.
I looked for spare parts for it in wrecking yards,but most engines that I found had the cyl head cracked from the valve seat to the pre-combustion chamber.
The turbo charger on my engine has a waste gate but I have seen others without any waste gate at all, the intake manifold is very restricted so I think that this will limit the boost. It is a Mitsubishi turbo with a three hole flange.
If I was you, I would look at the camshaft lobes, adjust the valve clearance, install an intercooler and do the same pump mods on it that they do on the VW's.
Exhaust manifolds tend to crack badly on the inside, check yours too.
You could make a bigger diameter pipe between the turbo and intake manifold.
Also replace your timing belt and balancer shaft belt.
Make sure that the cooling system is in good shape, overheating will crack your cyl head!
You can still buy new parts for them, try:
http://www.auto-parts-wholesale.com/Overall it looks like it is well build and it should last you a long time if you take care of it.
Hope this will help.
Herman