a decent SLR (like the olympus om series, the canon eos, and the nikon fm series) and a 50mm lens should do the trick. to go under 300, you will have to ebay though, since the local secondhand prices are typically higher.
OMs are generally great deals.
But there are heaps of other format options, like rangefinders, scale focus etc that you could consider. Those would fit well within your budget, but the tradeoffs could include stuff like non-changable lenses and less flexibility.
Praktica/Pentax Spotmatic + SMC Takumar or any other m42 lens you favour. Though, you'd probably want the Takumars. Many reviews online say they're better than or just as good as your Carl Zeiss Jena optics.
It's hard to beat.