Need something new

Hi chaps, I have a stunning 27" iMac, loads of memory etc bought new when I was doing lots of motion graphics work, anyway there's a strange pixelation issue with the screen, flashing square artefacts, they, of course, go away when I try and screen grab them. I guess it's a sign of problems, I have had it for over 7 years so I need something new. I do not need the real estate any more, the After Effects work has dropped off, I continue to use Photoshop regularly. I'm now retired and with no income finding cash for a new machine is impossible so I have to look at 'used' ... anyone any suggestions as to "bang-for-buck" - I'm happy to look at a big laptop or a smaller iMac ... anything else out there I should consider please ? (budget's not much more than £400) !