personally, i always go with thunderboltt 3 eGPUs because it futureproofs the performance in the hypothetical scenario I upgrade to a TB3 machine in the future.
you just need to use the Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter, you cant use any other cheaper 3rd party adapters because as of writing this, Apple is the only one that makes one of these adapters that is bi-directional.
in my experience assuming you're buying a new enclosure not used from eBay etc.
nowadays its easier to find TB3 egpu enclosure compared to TB2 ones, so the good TB2 ones are practically the same price new.
that said yes you need Apple's $50 Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter, but i think its worth it for the future potential even if it may be slightly more money.
as for the setup of your mac, its really easy there are tutorials to follow i'd recommend checking out egpu.io forums there are guides, comparisons of different enclosures and you can ask for help if needed.