One questionL Why 77-79? The 73-75's had D-Jet EFI as opposed to K-Jet which, in my opinion/experience, is easier to tune, setup and has fewer issues. Also, the 73-75's had the higher performance cams - in '76 MB switched to "smog" cams that cost the engine 30HP and turned it into a slothlike gashog (compared to its older more powerful brother).
I find my 4.5 VERY easy to work on but, as I said, it's got D-Jet which I find simpler and more straightforward (less parts to go bad, IMO). Other people HATE EFI and prefer CIS, but it's prefference I suppose...
The bottom end of these engines is as bulletproof as it gets, as long as it wasn't run 20k between oil changes or overheated in the past. If there was EVER a bottom-end rebuild (even just new rings) pass it by, chances are that it was either a: seriously abused, b: didn't need the rebuild, c: the rebuild used hard rings or cleared out the hatchmarks that otherwise would still be there.
The weak point is the timing chain guide rails. These are plastic in the newer ones (mine are aluminum-backed rubber, WHY MB didnt stick with that I have no idea). The plastic can crack when the rails get old and fatigued, or the timing chain stretches - as a result, the timing chain will skip on the cam gear and... well, let's just say that valves and pistons don't get together well when they meet
This is no problem, however, if you replace the rails (3 of them, and they are under $20 for the set). If the engine smokes at ALL, even if it seems heavy, don't pass it buy - it might just be valve stem seals (ESPECIALLY if the plugs have no oil on them). At 100-200k these should be replaced, and guides examined, depending on how it was maintained. Undoubtedly, if they are still original, age alone should say "Replace them"
Sorry if it seems a bit long-winded... but you'd love the 4.5's power in the 73-75's a lot more. I know I did, when I had the smog cams in my 4.5 before I found a good pair of the stocks, I got worse milage, ran hotter, and had less power (ESPECIALLY top-end!)