Is there a sweet spot for price/age when buying?

I want to get a used car and my budget is quite flexible. I'm used to old bangers, so something under £3500 would be fine, but I can afford considerably more, so I'm wondering whether I should go upmarket. Could it be more practical in the long term to get a newer car? Besides having a nicer car for a while, it should last me longer before I need to replace it again, and there's probably less chance of unexpected repair bills. I also wonder whether dealers take a bigger percentage of the car's worth for their profit, because if they used the same percentage no matter how much the car's worth they wouldn't earn much on the cheaper cars and it wouldn't be worth their while. Against that, depreciation is higher for newer cars. So is there a sweet spot?

FWIW it will mostly be used only for short journeys a few times a week and I'm looking to get something like a Focus 1.8 petrol.