An engine that always brings a smile to my face is a twin cylinder Mamod SE-3. They are fairly common and also tend to be quite reasonable in price. I think there are a few on evilbay currently at $150 or less, and a couple of them look pretty good.
You might want to check some out .... here's one that caught my eye:
https://www.ebay.com/itm/195970877698?
Yeah I’ve been burnt badly on eBay myself on other things.. and not the biggest Mamod fan. They seem junky to me, but that’s my opinion.
I wouldn't say Mamod are my favorites. Most are pretty simple engines, not very sexy but I wouldn't say they are junk. Granted the two I have (MM2 & SR1a) dropped into my lap but I had fun getting them going again. Luckily they were for free, so buying parts wasn't to big of a deal.
From time to time you can get a deal on a Empire - it would need a little help and likely paint (if bad paint bothers you). I been pretty lucky, I guess, buying off eBay. Only one time did a engine get to me damaged. Now I always ask the seller to remove parts (throttle, whistle topper, smoke stack) and either they do or they package the engine extra good.
I think you can get deals on Jenson and Wilesco too, just have to keep looking. Setup a eBay search to have new listings emailed to you. The good thing about Jenson, Mamod, & Wilesco you can still get new parts. Though it also depends on the age of the engine (the older the less engine likely the right part is still made).