Sorry to hear it's been such a frustrating situation for you. I'm just going to give my honest unbiased feeling on the situation.
Sounds like the seller is somewhat of a jerk, too bad you have to deal with him. But I don't think he is 100% at fault here.
First of all, this is a 1991 DSM, that cost $1800. No one should ever be expecting to purchase that, for that price, expecting it to be some sort of gem, regardless of what the owner said. I purchased my 1991 DSM for $2000. I'm looking at your list of issues, and honestly a lot of what I see on that list, I'm not suprised to see from a fairly stock 1G that simply wasn't maintained too well (which is what I'd expect of any DSM). Can anyone really say most of those issues are total shockers? Sounds like a bunch of maintenance issues that I would expect from an 18 year old vehicle that hasn't had a lot of TLC and attention. Mine had a good 90% of the same issues yours has when I bought it. Plus a bunch that you haven't mentioned.
- brittle wires / wire connectors
- No idle stop switch
- Tranny/clutch issues
- Oil leaks
- Vacuum lines capped off with screws.
- Worn Ball Joint.
- busted exhaust manifold studs
- Injector Seals Bad
- Bad Knock Sensor
- Missing timing covers
- Bad radiator
Yes in a perfect world the seller would have told you all the issues, but not everyone will be that honest. It's too bad, but that's the real world. That's why you inspect it, bring someone with to inspect it, test drive it, etc. Don't get so worked up, it's an old DSM, you just got a crash course on what it's like to own one. You need to get yourself a Haynes manual, and start learning to work on it yourself. If you can't work on it yourself, expect to be dumping a lot of money into it because little things go wrong here and there on these older cars, especially if you go have some fun in them and start modding them.
Good luck though, and if you ever need help ask. We're all here to help, so don't be scurrd to ask for some help.
OK that's it from me