Egpu can also use mpcie and expresscard. Expect the adapter to be ~$113 plus the price of the 7770 and psu. You're going to have to be more specific on the laptop model to know if your connections would even be able to be accessed without having to take apart the entire laptop.
You don't need an enclosure. But you are right about not being worth it. You could sell the old laptop and the money you would spend on the adapter+gpu+psu would get you a laptop that had a better gpu built in.
It's just not worth it. Gaming and laptops have never really gone hand in hand that well when compared to desktops.
If you need mobility, save your cash and make a mini-itx gaming rig - that's your best bet.