One thing that would really help would be if you could post some good detailed photos of the underside. Sometimes things can look worse than they really are (then again sometimes they can be worse than they look as well
Several things to think about as far as the rough ride goes. The early 20' Argosy motorhomes (like yours and mine) did not have the later air bag rear suspension, instead they rely on the rear springs for the cushioning effect. The spring leaves are supposed to slide on top of each other as you hit a bump. As the motorhome gets older, corrosion sets in and the individual leaves slide less and less until they reach a point where there is almost no movement at all. This is one of the major causes of rough rides.
There are at least two things you can do to fix things. One would be to buy new replacement leaf springs. Yes they are available and I will likely be doing the same thing, and install them. The other would be to install helper airbags which we will be doing as well.
For a point of reference, on a lot of the older vehicles there were leather "sleeves" that you could buy that are wrapped around the leaf springs to protect them from water and dirt. You were expected to apply grease to the leaves and them wrap them. This allowed the springs to be lubricated which made them slide easier and the sleeves kept the grease clean. There are companies that make these sleeves or if you know how to work with leather you could make your own. Somewhere I have some information on what they would look like.
For the front suspension just figure on rebuilding the entire front end. Replace all the tie rod ends, ball joints, coil springs, drag link and bell cranks. Also install the airbags that go inside the front springs, they really help the ride. The difference in our 310 without and then with bags is like the difference between night and day.
The thought of installing a "donor" chassis is not for the faint of heart and would likely be cost prohibitive. Something to consider only as a very last resort. And that would only work if you had LOTS of money to spend.
I would consider the engine problems minor at this point and wouldn't spend a whole lot of time or money on them until you're 100% certain the rest of the motorhome is worth repairing.
Looking forward to pictures.