Do you know any good prebuilt that I could upgrade for under 300?
Or shall I just wait for covid to go and then build a pc?
Personally, i'd suggest waiting, until you can afford to spend a bit more. If you have 300 to spend, you're going to end up buying a system that's using an older generation CPU and possibly won't perform as well as you want to. Ultimately, you'll be using an old socket on the board and that will severely restrict your ability to upgrade to any of the newer generation CPU's without also having to replace the motherboard.
If you only have 300 to spend, and you don't plan on upgrading any crucial part of the system any time soon (CPU, motherboard mainly) then it's not as much of an issue, but if you want something you might look to upgrade in a year or two, you're probably better off just saving up and getting something better further down the line. 300 doesn't buy a lot these days. A lot of the newer high end CPU's cost 300 or more alone even at RRP/MSRP.