http://MacSales.com (OWC) is still the most-recommended supplier here, because they have good return policies and a lab full of Macs. They pride themselves on their service, so they do not sell stuff that is so "difficult" it needs a lot of hand-holding to get working.
OWC sells their own private-label SSD drives, which are pretty good and reasonably priced, and you know they will work in your Mac.
Look for the "My Upgrades" link on their main page, and it will lead you to parts that will work in your specific model Mac.
They offer two different genuine Thunderbolt Docks supporting up to two displays are shown on this page:
https://eshop.macsales.com/shop/thunderbolt/thunderbolt-docks
Most often posted as NOT working properly on a Mac: DELL Docks -- Trouble Prone, Unfriendly, and NOT working properly on a Mac. Most require Windows to work at all.