The WR500s weren't registerable - although a fair number of of old IT490s got big make-overs around the time the WR500s were released....
Really, with a good rider, any of the long travel bikes (say, 1980 or newer) can keep up with most trail rides. But for most of us, double discs and water cooling will make life easier.
Stuff like the linkage-rear IT250/490 feel VERY old when you're riding them - they steer like boats and are top heavy. At the start of the day, you will be able to annoy the hell out of everyone (by keeping up with their new bikes), but at the end you'll be working a lot harder.
As a rule, 500cc 2-strokes are over-priced. Oh, they're fun to belt around on, no doubt about that - but living with one all day in the bush is a pain. They stopped making them for a reason! CRE500s in particular, are worth silly money (if you can find one) - sure they're a cool bike, but for $11k for a reasonable used example, you may as well buy a new TM300 or something.
The Husky WR360 is a good thing, although the old ones (early~mid 90s) seem to be worth just as much a the last of them (early 00s)... I reckon the early ones are over-priced and the late ones are a bit of a bargain. Ditto WR250s in the same age-range.
The last of the RMX250s is a hugely under-rated bike, and $3k ought to get you a really good example. With not too much work, you can turn one into a registerable 96(?) RM250 - and blow most 450Fs away...
The rego-able Yammie WR250s are hard to find, and usually over-priced (compared to the RMX anyhow). Its rare to see one for under $3k, even average old examples...
The '04 YZ250WR is a cracker of a bike, but are holding their value very well. Again, I think they're over-priced.
The CRM250 is too soft-core and always seem to sell for silly money anyhow.
Any of the perimeter framed TM250/300s will give you a ton of bang for your buck, and are usually maintained properly (making them less of a lucky-dip when buying an older example). The 250 in particular, can be pretty brutal, but nothing too crazy and there are ways too tone them down without turning them into an XR250...
The early~mid 90s KTMS are also hugely under-rated (before they went to the current dark orange). Everyone I know who's owned a 96/97 KTM250/300/360 has raved about them, and even good ones sell for ~$2k when they come up.
The water cooled KDX250 is a big fat pig of a bike, but can be made to go well if you chuck enough aftermarket parts at them.
TS250 is too soft core + old.
If it was my money, I'd be looking for an RMX or KTM, while keeping my eyes open for a Husky or a Yamaha WR250. There's also been a few complianced-plated late-80s YZ250s floating around lately - if you could get one for the right price, they could be a good option too (ditto the WMWs are Middo mentioned).