My problem now is I’m using a really old computer
Have you considered installing linux on it? I am using an old-ish laptop that was going really slow with Vista and has been running fairly well with Ubuntu. It doesn’t run like a new computer, but Ubuntu has kept me from buying a new computer for about 5-6 years. If your computer is running XP, try lubuntu.
I run Darktable, Raw Therapee, and Gimp and am quite happy with the price. 🙂
Having said all that, if you want to buy a mac, buying refurbished can save you a bit.