Aside from my Mac Pro 5,1(2010 upgraded to 2012 CPUs), I am a daily heavy user of a 2012 15" "classic"(non-retina) MBP, AKA MBP 9,1. This is a model with a bit of a cult following-the pre-Retina form factor was the last with a built in optical drive, if that's your thing, or when you come to the point of realizing that you haven't used the ODD in a couple of years you can inexpensively add a ton more storage space. The computer mostly operates off a 1tb SSD, but I have a 2tb spinner in it for big files I want with me but where I don't necessarily need fast access speeds. The 2012s specifically are the only ones of this form factor with USB 3.0, and the 15" 2012s are among the few dGPU computers without GPU issues. This was also the last model available with an anti-glare(matte) screen, which I have.
With that said, I've been traveling a lot more lately, and that computer is a pain to lug around. For that reason, I bought a 2015 13" Pro that I primarily use when traveling. There again, the 2015s are sort of the last of their kind-they have the good feeling and reliable scissor keyboard, no touchbar, and USB-A ports(plus other useful ones like HDMI and built-in SD readers). These have PCIe SSDs that you can replace yourself-I actually saved some money by buying one with a 128gb drive and buying a 512gb separately(the 2015s can use NVMe natively-I'd have actually gone that route as opposed to an Apple OEM AHCI but had bought the 512gb in anticipation of a deal for a 2013 that fell through, and NVMe is supposed to be buggy in 2013s). I REALLY didn't appreciate just how good the battery life is until I actually used it, but it blows me away-I can reliably get 5 hours out of it, and can stretch that longer if I'm really careful. The drive especially makes it FEEL very fast, although if I'm really hammering the CPU my 2012(which is a quad vs. dual) still get the job done faster.
The 2015 iGPU 15"s are good computers. In fact, I helped a few folks at work spec them out when they wanted an alternative to the USB-C only touchbar models(for a while they were still available along side the TBs). When I was shopping a little while back, though, they were running a lot more than $500 for good ones. $500 was more in the range of 13"(in fact that's what I paid for mine). I'd suggest getting 16gb of RAM to future proof. In addition, you want one with the best battery health you can find. The pre-Retina MBPs and also the older non-retina MBAs make for a relatively easy DIY battery replacement as they are screwed in(you often need special screwdrivers, but those aren't difficult to find and if you buy replacements from Newertech, you get the correct drivers). The non-Retina machines usually run ~$100 for a quality aftermarket replacement(Newertech) or ~$130 for Apple to install a factory battery if still available. The Retinas are a different story-the battery is glued in, and while DIY replacement is possible it's not for the faint of heart. Newertech will sell you the battery for under $100, while Apple charges $200 to do it for you. I would avoid buying one with a bad battery unless it was priced to make replacement worthwhile.
