[caveat emptor here — I'm a relative newbie to all of this, so... probably listen to David]
This is an interesting conversation — I had only just looked at David's setup and was shocked to realize that he's running the current LPX on a 2013 i5-based machine!
I'm a professional designer, so I have the "luxury" of already requiring a much stronger Mac, but even with my "baby LPX projects" (I just started with it a few months ago) I occasionally hit playback snags. I think that just demonstrates how someone with David's expertise knows how to avoid *needing* more computing power.
Anyway, I'm mainly just chiming in to extoll the benefits of the Retina screen. I'm totally biased, being a visual designer who really relies on it, but even in LPX I can't imagine "not caring" if I have the option — the UI just looks so much more crisp at that resolution, and tiny details like the Mixer level numbers are a lot easier to read.
I'll also throw in that if you're going to put anything on an SSD, I would think it would be the project files instead of the Logic libraries. I actually run the latter on a traditional HD to save space on my SS memory — it takes several seconds to open the project, and flipping through software instruments can be a bit annoying, but I'd rather have the performance disk devoted to project playback. [Edit: but you can always copy the Logic files for just
the project you're working on to your SSD as well]
Sorry, one last thing: I've been a professional designer for over two decades and have never bought a new Mac. Yes, used computers are a risk in theory, but I've had lots of them and have really never had a dud. I do recommend eBay over Craigslist since you can see the seller's rating and have some (maybe) recompense available if the seller has screwed you over.
Okay, now I'm done.