Or do I consider a used Mac Pro, and add internal drives and upgrade what else I can?
Of course a third option would be to go with a new mac pro, but at a minimum that's twice what my budget allows for now.
I've cut down your choices to these 2. Personally, I think editing, cutting, and exporting in Premiere Pro on a Macbook Pro is just an exercise in futility. Or at least: an exercise in extreme patience. It's not worth the trouble, IMHO.
New Mac Pro absolutely and without question. That assumes a budget. They're not cheap, but they work well with Pr.
Old Mac Pro as a stand-by with the fastest (clock cycles) chip or chips you can find. In this specific case, don't get caught up in the "higher core count" race; look for higher clock cycles. Upgrade the video card to an nVidia 700-series because it'll likely come with a piece-of-junk AMD. And pack those 4 drive bays with SATA drives.
I was editing with a Mac Pro 5,1, which also had 2 extra SATA plugs for the 5.25" bays (CD/DVD). I put a 2.5" SSD in the second bay, with an adapter so it'd fit. Then I put a bunch of 1TB drives in the 4 drive bays. I made 2 2TB RAID0 volumes for media and output files. The OS and apps went on the SSD. It worked fairly well, within the limits of the hardware (Westmere chips) in it.
Jason, thanks for sharing your experience and helpful suggestions. Specs such as clock cycles are new to me.
I did just now buy myself some relief on my laptop: discovered that the CUDA driver hadn't been installed let alone enabled. I did a quick test with a familiar file, re-rendering previews and then exporting. Render times were significantly shorter and export seemed faster too. Not as radical as render, but that maybe due to a slower spinning export drive. Lots of hardware specs to track, it seems, when it comes to even small scale video production.
I can see though that an upgrade worthy Pro, new or used, will be the way to go eventually.