There's plenty of good advice here already, but I might just emphasise one point that has already been made.

The camera that you buy will be old and possibly heavily used. It may or may not be reliable. Unless you are desperate for money, and your budget suggests otherwise, I would be sure to buy from a dealer and get a guarantee. You don't want to find yourself in a situation where you can't do your work because the camera has broken down.

If you were a hobbyist you could enjoy taking a gamble on Ebay, and I have both won and lost doing that in the past, but this is too important to you.

In addition to the names that have already been given I suggest that you consider FFordes, they have been around for a long time and have a good reputation. I have used them in the past and have been able to return a camera without problems.

Finally, you could add the Pentax MX to your list. It's a nicer fully manual camera than the K1000, but tends to be a bit of a cult object and therefore a bit more expensive.

Best of luck with your studies!