For reliability mercedes and BMW seems a bit meh to me. They always seems to get issues but it's random. With any car as long as it's been taken care of (Don't take the sellers word for it you'll see it through reciepts) you should be ok.
My opinion is if there's less parts you'll be better off. So getting a car with no turbo and is very simple, not too much tech etc. Which is why i love my yaris! Toyota and honda do seem the best, but the problem are people stealing exhaust parts off of them. Not much of an issue on toyota, as long as it's not a hybrid. For complete reliability for cheap a petrol toyota non hybrid would be the best imo, followed by a diesel toyota and honda. Diesel seems to be stronger than petrols. My uncle recently passed his mot on a 2014 diesel yaris and it's got 275000 miles on it. From a reliability perspective if I had £5000 but could stretch to £7000 I'd get a euro 6 diesel civic, would cost you peanuts to run, in fact might even be cheaper to own than your corsa if you keep it for years.
As with most cars, you'll have clutches and alternators and head gaskets etc. Just make sure it's in good condition. Check under the oil cap for any mayonnaise, it should be at the fish and chips, not in your engine. See if you could pull off in 2nd gear just using the clutch on a flat surface. Check tyres.
I know I mentioned reliability a lot but I'm just speaking from what I've seen. I'm doing a level 2 course to become a mechanic and I do work on cars in the college workshop. Out of the many fords and vauxhalls there are, the r reg honda civic BY FAR is the easiest to work on. And I personally drive a yaris and so far it's been good.
You've mentioned you're a second year student with a "lack of a proper full time job". Given we're in the middle of a pandemic and the future is uncertain, it would probably make the most sense to drive your corsa until it falls apart. If it's an old one worth less than 1k then drive it to it's death. If it makes it to it's next service buy a socket set and you could probably service it yourself. Saves you labour charge and you get tools to keep.
However if you do want a German car I think the best would be a 320d!