So I plan on getting a 27" iMac 5k, but on a budget. I will probably bump ram to 16gb but when it comes down to hard drive I'm not sure which I should get, since I intend on buying a USB 3.0 SSD enclosure, SAMSUNG 850 pro for example. If I go that route to boot up the computer, should I just buy the 1TB model or opt for the fusion drive if all possible.