Bus market is hard to read, especially since covid hit. Covid seemed to have brought out a lot of unknowledgeable buyers. It's why you see em priced so high and goofy right now. Pre-covid, your high sellers went to your exporters, which were the dt466's, 6bt/isb and isc. That left a lot of your buses, those powered by any v8 or your smaller hp I6 and your cat engines pretty cheap. As covid hit, prices rose, buses for sale became scarce, the top of the line buses sold for $10k+, leaving people buying the cheaper units at 2-3x what they were worth. Some flippers also caught wind of the hot market and a few of them dabbled in the business, and got burned in the process.
It will settle out. Covid is almost a thing of the past, and so are quarantines. The whole landry/petito thing gave a lot of people second thoughts about the van/bus life too. Living with someone is hard. Way harder if you're crammed into 40 total square feet 24/7 miles away from home. You better be pretty square with the person you're with.
The idi's in a bus have a bad rap for several reasons. For starters they were the entry model, low power, low optioned bus. Then throw in all the problems with the idi platform, like air intrusion, glow plug systems, etc. along with the at545 issues. Then combine that with how stupid and hands off society has become with vehicle operation and maintenance, and you can see why people are told to avoid them. Most can't figure out if they have dead batteries or a bad starter on a no-start. Throw a bunch of other **** at them and they'll quickly lose interest.
Your trans modulator might have broke and is stuck in full throttle position. It'd give you normal shifts accelerating, and hard shifts decelerating. It's not the end of the world, but it shouldn't downshift hard, at least mine doesn't. Any c4/dex merc fluid will work. You don't need something expensive, and I wouldn't use anything low viscosity like a tes295 fluid either.
Trans swaps, especially upgrading to an electronic one, are thousands. I was eyeing either a pumpkin or axle swap in the 4.5-4.8 range. Or going with a auxiliary trans with and overdrive. With an "underpowered" engine, I felt going with a faster rear would lose a bunch of acceleration, which is why I was looking for auxiliary transmissions or a 2 speed axle. I've got the same f170s/ra39, and your options are pretty limited. Ideal would have been for me to find a cutoff assembly with air ride, 2 speed axle, and 4.56 gearing. Even if it was a 30k lb axle, with the shop I could about make anything work. Probably have to ditch the dayton wheels, but I'm indifferent to that.