Upload & Sell: On
I Would: Stick with your current MBP, putting your images files LR5 catalog(s) on an external drive. As your catalog gets large, you'll observe some performance drop off. Upgrade to a better machine when performance becomes rate-limiting. Going to an iMac next makes the most sense to me, dual monitors if you want. If portability is an issue, you're Mac options leave you with a newer MBP which, on a bang-for-buck-basis, is pretty expensive. The iMac is, on the same basis, a fine deal.
iMac screen calibration works fine, but is an issue you'll need to read about.
I buy external drives from OWC:
The suggestion to build your own Windows machine are more than valid, I am sure. The thought of using my time that way is out of the question, in my world only, and I dislike Windows enough to happily spend money avoiding it.
Your experience, and that of others, may vary of course.