Most of the difficult work on any engine rebuild is in the cylinder head. If you can't do that yourself but can get the head off, take it to a shop and then you are just paying for the work they do on the bare unit. You can measure the head distortion with basic tools. A thick piece of plate glass and a set of feeler gauges will get you close enough IF you cleaned off the mating face properly. In the shop they will use a surface table, and feeler gauges, but only because they have one
(a surface table is a heavy piece of cast iron (or maybe granite) that has been ground very flat) Specialist shops may use fancy depth gauges or even a laser scanner to achieve the same result, more accurately.
The rest is fairly straightforward if the crank bearings are okay.
If the rebuild can be done "in-frame" it means that once you drain the oil and remove the sump, the crankshaft with the pistons attached must come out from the bottom (It will be heavy, lower it on a transmission jack and make a note of what cap goes where, and which way round).
You don't need much in the way of special tools. For example, a ring insertion tool can be made from a baked bean can and some worm-drive clips. Anyone can use plasti-gauge to measure cap-bearing clearances.
The problem you will have is that even the best service manual will not teach you how to use the tools. If you don't have basic mechanical knowledge and competence you stand a good chance of making it very much worse.
Re-assemble a crankshaft wrong and you won't necessarily be able to tell when you start the engine. You will be able to tell when it does several thousand dollars of damage to the crank and block.
Diesel engines are fairly simple beasts, but they are big and very heavy, and not for beginners unless you can get a knowledgeable friend to steer you through the process.
It's simple stuff, but not necessarily easy ... those are very different concepts.
Automatic transmissions, on the other hand, are not for the faint-hearted and with 40 years experience wrenching on engines of many kinds, I'd pay a specialist for that. Torque converters, which is a scary term, are relatively easy if you can get the transmission out.