The Sencore stuff and ST1000 are solid state but they are old and unless you find one that someone's already gone over, I'd plan on refurbishing/repairing it. (Capacitors, and there are a few mods to improve them, and in the case of the SG165 there's some plastic stuff that breaks, lots of great youtube videos and forum posts on both topics) In my opinion they are overpriced for what they are, since that's what everyone thinks that's what they need to buy. They don't seem to come up for sale all that often (well, the SG80 and the ST1000a). Oh, and if you get either of those get yourself a frequency counter as well...
I got my HP 8904a (with the correct serial# and option 001 for FM stereo) for about 250 shipped and the 8644B for around 350 shipped in the last month, to give you an idea.. where as the ST1000A's have been going for 800+ on ebay lately... even the SG165 is going for 500 bucks in working condition. You can also pair the 8904A with all sorts of other high end signal gens like Marconi, Fluke, other HP's, etc... which also sell dirt cheap. So, you could get a setup like this for 5-600 bucks.
Most of the time this type of stuff (the HP type equipment) is sold from eWaste scrappers, and you can lowball the heck out of the best offer price. Also, many of them list the stuff as 'working condition' (not for parts/broken) with a 30 day warranty, and if often arrives broken, since they don't really know how to test the stuff properly beyond powering it on. Then when you take them up on their return policy they just tell you to keep it and refund all your money since it's not worth the hassle for them. I've had this happen a few times, and was able to fix the gear easily.
I doubt you'll find another AVR-5510-PI for sale, I've never seen them before the guy who bought them from the Pioneer auction put them up for sale and there's practically no info available online. Too bad, they seem great.
Kenwood makes some too. Here's one for sale at what I'd consider a cheap price. I don't know much about them, though.
Kenwood SG-5110 FM Stereo FM-AM Signal Generator 100KHz-110MHz | eBay
Good luck and have fun!