One of each
I think I'd try for one of each of the main categories:
1 - The classic 70s/80s Manual SLR
Any of a wide range of the quality consumer grade SLRs capable fo full manual control from the 70s or 80s. Canon AE-1, Nikon FA/FE/FM, Pentax K1000, Olympus OM-2SP (or 40). You can get a clean example of any of these with a 50mm F1.8 for under $50US. You'll probably get a bag, second lens, and a flash included.
2 - A fixed lens range finder
Again, my preference would be something capable of complete manual control (Canonet, Minister, etc), but any of the semi-automated ones from the 70s (electro, Hi-matic, etc) would also be good.
3 - A Medium format folder
For the price it'll probably be a fairly basic unit from the 50s, but it'll be clean, work well (within it's limited shutter speed range) and take outrageously sharp images (Nettar, I'm remembering you). It'll let you see the majesty of a big negative, and make you work for it (no light meter, and God those view finders were tiny). Bonus points for finding a good working coupled rangefinder unit in the price range, bonus skill points for taking in focus portraits with a shallow DoF on a scale-focus unit.
4 - Something Lo-Fi
Maybe a toy camera (Lomo, Diana, seagul, etc), or maybe just something really clunky (Argus C3 brick, low end TLR with very limited aperture and shutter speed options, FSU Rangefinder in good working condition if you're really lucky). The point is to have a camera that you just throw film in and have a complete laise-faire attitude about. F8 and shoot from the hip.
5 - 90s AF SLR
I'd love to end this with a modern Medium Format, or even Large Format option, but I really haven't seen anything that would fit under the price limit. So I'm going to say try to get something that's as close to modern as possible. Canon EOS Rebels and ELANs are a dime a dozen, Nikon F60, Minolta, Pentax whatever you can find. Bonus points if you can share your lenses (without adapter) with your DSLR rig. See just how far DSLRs have (or have not depending on your point of view) come. Listen to the industrial glory of that screw drive AF motor and film winder.
For under $300 you could have a complete collection that lets you sample 70 years of photographic toys/engineering-marvels. Each of which will take a different mindset to use, helping spur your creative juices.
Full disclosure, I don't have anything from #5 in my collection.