check out the site "backmarket.com" they sell refurbished or renewed macbooks (with a warranty mind you) for very good prices, depending on the exact specs 500 bucks can get you anywhere from a 2008 to a 2017 macbook, IM NOT KIDDING seriously check it out.
Keep in mind most Macbook Pro's storage is upgradeable these days aside from the newest ones (i think 2018 and on) so don't worry too much about not getting a 1tb storage option, its okay to get the smaller ones for a better price then upgrade for an even better price lol
But honestly I recommend 2015 13" or 15" (because they have an hdmi out which for me is pretty useful) Which all of them are generally around $500 on back market
I like the size of the 13" , but if you absolutely must have the faster PCIEx3.0 speeds (storage speeds), settle for the 15" 2015 or the 13"/15" from 2016
You can't go wrong with really any of em, they still perform well today.
Good Luck!!