Make sure you get a usb enclosure that supports UASP. This is pretty crucial if you’re using it for a boot device as it’ll give you much better read/write performance and doesn’t just treat the storage as generic memory, and carries some sata protocols.
I used this one https://amzn.to/2UrtKIX for over three years daily in a production environment without issue - booting an iMac and running creative suite.
Then you want to put a really solid ssd in there. I’ve had good luck with high end Samsung’s (830s, 840 pros, 850 pros...) but will let you do the research...
You can get faster speeds with the latest nvme enclosures, just make sure to look out for UASP so you don’t just wreck a nice disk.