the way my workspace is (patio door to my left) the shiny reflective imac would drive me crazy, unfortunately they don't make it non-glossy or I'd have probably gone that way.
but, I like to have two screens, so I just got me a MacMini (the time to buy one is when you need it, can't wait for if/when Apple might make changes to it) and two Dell 27 inch screens running at 2560x1440 resolution. I went with the top of the line Mini, 2.6GHtz i7, added 16 GB of memory myself (crucial brand, takes a minute or two and anyone can do it).
to run two screens at this full resolution (which online mac store chat AND in store techs told me is no problem - a lie) I had to get a dongle that turns one of the 3 USB 3.o ports into a display port, little "external video card" by StarTech (got via Amazon, but check their site for info) and it works like a charm. I'm just used to having two displays from my macbook pro with extra screen I just retired. LR supports dual screens, I can either have the grid on there or a 100% zoom or just regular size while zooming in on the other screen where all the tools are. Works like a charm and for photo editing I'll never even get close to the limits of the machine, it's not that much computing, compared to say 3D rendering or video, but even for video it'll be just fine. Much cheaper too, all in all about $2500 or so. Close to $700 less if you don't need two screens.
The dell screens are matte, already nicely calibrated for all I can tell, haven't plugged my calibration tool in yet. Both look exactly the same if I have a photo overlap onto both screens.
iMac is great, definitely less cable salad, just can't deal with the shiny surface.
Neither can really be upgraded I believe, but I'm not concerned about that. I don't play games on it so a faster video card etc is not an issue I'll ever encounter. I have all photos on external drives, I doubt I'll ever fill up the 1TB hybrid drive I got installed. Added a USB3 hub to have more than two inputs. (Dell screens also have usb hubs, not sure if usb3 though, and I prefer a box on the table instead of cables hanging out the side of my screen.
Very happy so far, snappy fast.
Anyway, just what I bought after looking into this since last summer, laptop was starting to choke on the 5D3 files and at close to 7 years old it's an antique.
I run LR, PS, assorted plugins from OnOne/Alienskin/Nik and everything is fast and smooth.
this will easily work for quite some years to come, files will get bigger again, but not until I can afford a new camera and as far as I can tell, the mini will not choke on larger files either. Not talking PhaseOne ditigal backs though, LOL
Oh, the screens are Dell U2713HM, I'm very happy with them.