There's a lot on there alright.
I currently have a (I think 2011) MacBook Pro which runs great still, but I would make the cutoff around 2010 to be safe.
They should all still take El Cap (the latest OS) without issue. If you go with a Pro, you'll be getting a noticeably better laptop. Obviously if you stick to 13.3" you're going to be getting the most laptop for your budget.
All that said, you can probably buy a brand new and better laptop for $500 if you avoid Apple. Apple is a luxury brand, it's not really for a budget, and certainly not for anyone who wants something great on a budget (even used).
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