If you get a 21", MAKE SURE that you get one with enough RAM -- because adding more later requires disassembling the iMac.
If you plan on "booting externally", do it right the first time by getting a Samsung X5 instead of a t5.
Connected via thunderbolt3, the X5 will give you 4-5x the speed of the t5.
I'd really recommend that you get an Apple-refurbished 2017 27" 5k iMac with a fusion drive (at least).
In that case you can get 8gb of RAM and add more yourself later... IF you find that you need it.
The fusion drive (at least the 2tb version) will be pretty "snappy" right out-of-the-box.
This way, you get a very usable Mac from the get-go, and can add the X5 later.