Help me buy a MacBook Pro.

It's worth noting that even after the flip to an all-Retina lineup, the old machines are still available as refurbs for a long time. They're still selling refurb MacBook Pros introduced in February 2011.

(i.e. most of the time things will be on the SSD until I need more space, and then I will move the media I never look at onto the external.)

Okay, I'll be getting a USB3 enclosure then. Is there a significant difference, in this scenario, between 5400 and 7200 on USB3? I have v. little experience with externals, so I know it makes a big deal when it is your main HD, but I'm wondering if it is even worth paying for the 7200 if I'm going to be using it primarily for archiving.

A 5400-rpm drive with USB3 will be way, way faster than a 7200-rpm drive with FW800. The interface is the limiting factor, in my experience. Thunderbolt should be faster still, but thunderbolt enclosures are as scarce as hen's teeth and disproportionately expensive.

If I'm going to be swapping out the internal hard drive for an SSD and using the swapped-out drive as an external hard drive (in an enclosure) will 5400 vs. 7200 make a big difference? I know it does when you're loading the OS off the drive, but I'll only be using it for storage. I am not 100% sure where the bottleneck will be, nor what the best interface is (Firewire? USB 3?)

No, the tiny bump in CPU speed won't matter. I strongly suggest getting whatever is tops in the video hardware though. Apple's entire screen rendering system makes agressive use of the GPU and they'll be pushing that even harder as time passes. You may be unable to upgrade to future OSes if your GPU isn't up to snuff. Refurbs: I've bought 2 of them and I'm not a big fan. Both were reliable but you aren't saving all that much money and both had sufficient case damage that the machines were just weird to use. In addition, you get what they have instead of exactly what you custom order. Your own SSD: Up to you. If you do install your own, you'll need to enable TRIM support via 3rd party hackery. It's not hard but don't forget. Another option with the class of mac you're looking at is to keep the hard drive in there and swap out the optical for an SSD in a http://www.mcetech.com/optibay/. Your own memory: I haven't done this on any of my Macs. I buy them maxed out. It's not worth my time and the possible risk of damaging new hardware. I bought the high-res matte screen and love it. It's totally a personal preference thing though so no one can really answer that for you. The current generation of non-retina MBPs are actually pretty easy to crack open and work on. The previous ("bath-tub") versions were a nightmare though.

They discontinued the 17"? That sucks. But speaking as an owner of a 17" MacBook Pro, it is an awkwardly large computer. For example, if I want to use it in my car, I have to get out and go sit in the passenger seat, because I can't open it in the driver's seat.

Opening the Macbook Pros, even the older not soldered shut ones, is not a trivial task I keep a few around for spare parts for our home jukebox and find them relatively easy to open, but I would recommend that DIYers use the right jeweler's drivers as the screwheads are easy to strip.

I've seen tons of technical folks with 17" MBPs, and they are not the type to put up with inferior hardware. I'm almost certain that Apple's discontinuance had more to do with the positioning of the 15" retina model as the top-of-the-line than anything else. Manufacturing those high-res screens is very tricky (defects are proportional to panel size and pixels), plus getting a graphics card to drive the pixels but still give 8 hour battery life is another challenge. Doing all that with the larger, higher resolution screen in the 17" was just too much. So, Apple dropped that model rather than have an awkward lineup where the 15" retina was the flagship despite a 17" non-retina model that was in the same price range. My completely amateur guess is that we'll see the 17" return next year in both flavors.