iMac vs Macbook Pro?

Hello! I want to emphasize that I'm a hobbyist so I do NOT need top tier specs.

I recently got into making drone videos with my DJI Mavic. I'm new to Apple products, video making, and don't know much about computers+specs. I do plan to purchase an iMac or MBP to edit 4k footage on Final Cut Pro

1) I was considering a 21.5" iMac, but after doing research a lot of people told me the 21.5" is very underpowered even if I customized the best specs on the Apple website. If I were to get the 27", what specs (i5 vs i7, processor, RAM, etc.) should I consider for 4k? Again, I'm a hobbyist and my price range is preferably <$2000, but I'd like to know my options at a max of $2500.

2) I've seen a lot of review stay the 2016 15" MCP+touchbar is not worth the price tag; what are your thoughts on the 2015 15" MCP? Capable to smoothly editing 4k w/ FCP

Apple refurb store has a 2015 15"; thoughts for 4k editing?
     
Refurbished 15.4-inch MacBook Pro 2.2GHz Quad-core Intel i7 with Retina Display

Originally released May 2015 - $1699
15.4-inch (diagonal) Retina display; 2880-by-1800 resolution at 220 pixels per inch
16GB of 1600MHz DDR3L onboard memory
256GB PCIe-based flash storage
720p FaceTime HD Camera
Intel Iris Pro Graphics

3) If the 2015 is capable of doing the job; which 15" model should I get? Its hard to know the available models since the Apple website doesn't sell them anymore.
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Thanks! I'd appreciate the help!

Edited by c0t0paxi - 15 Apr 2017 at 3:55pm