Personally, when I need to use a Mac I configure it for what it ultimately is - a looks-first trinket, so I don't like having multiple cables strung out of the rMBP's. Sure it's capable of doing what you need, but an iMac will:
a) take up less space on a desk than an rMBP + Thunderbolt display as well as look tidier
b) Look considerably tidier than a third party monitor + rMBP.
You get a desktop CPU with an iMac - however the bottom-feeder 27" model is probably only as powerful as your bottom-feeder rMBP15" from 2012. Add to that the fact that cooling isn't necessarily any better for intensive duties than an rMBP - my 12 & 13 iMacs idle significantly higher than any (even far more powerful) desktop I own, bar the XPS 27 - which are comparable machines and run only slightly cooler, but obviously in Windows I don't have to make a casual-kitchen-use-grade computer a working PC, but rather leave'em where they belong.
The memory is the only thing you can upgrade on an iMac.
But if you're going to get an iMac, I'd get something that would be an improvement on the notebook, i.e. not the bottom-feeder.