I also need to get a 300mm (ish) lens at a fair price so any suggestions would be appreciated.
Personally, I would get an 1800's Kershaw or similar shutter and a barrel lens.
When you shop in the antiques section, there are some lenses with a definite personality that can be had for much less than their modern equivalent.
Also, having a separate shutter could allow you to experiment with optics (which is always the cheapest way to do things).
You could also get a Packard Shutter. There's currently one on evilBay for a decent price.
It will need a new hose and bulb, jut that's something that can be hand-made easily. It also probably needs a bit of oil, but these things are so simple that it's no problem.