I run an imac 2013 i5 off a usb3 attached samsung 860 evo 500GB and it absolutely flies and just use the internal 1TB internal drive for large files. Only trouble with this is that it will only run high sierra and not mohave (although not really missing much) because if you try to run mohave off the usb3 ssd it takes about 10 minutes to boot. Something to do with the new filesystem but i reckon Apple have crippled something so you can't use the USB3 SSD. Now wheres my damn tin foil hat....
Some do, some don't. It is a bit silly though that they don't just have SSD's, if their drives were user replaceable it wouldn't be so bad but the way that they are locked in does make it annoying.
"Intel Core i5 3rd Gen"?! I hope that's just an obvious really bad typo and the rest of the spec list is correct, or the recipient is going to feel conned (the mid 2017 model should come with an Intel Core i5 7th Gen CPU, the I5-7360U). Showing as out of stock, for me. Thanks for sharing, though.
I believe they have fusion drives
That's a 2015 model, the one being sold here was a 2017 model for only £30 more and with a faster processor as well.