Is it worth replacing the lid for a new one with a relief valve?
No, you can use the "gas in" for a pressure relief valve by pressing in the poppit
Should I swap out the barbs for MFL disconnects?
I have barbs, and I like it just fine. But you could if you want.
What's a good fridge for turning into a 2-keg 1-tank keggerator?
I think there are lots of different ways to go, from a dorm fridge to a full sized fridge or even a small chest freezer depending on what size requirements you have. I have one small dorm one (for the cottage), and I have one full sized fridge. The full sized fridge is certainly more convenient- I store hops in the freezer section, yeast on the door, etc. I used to have a small "apartment" sized fridge and that really was ideal as it was bigger than the tiny dorm fridge but not all that much bigger footprint wise and the c02 tank fit in it nicely as well so it was "clean" looking. It died, though, so I replaced it with a full sized fridge- as a full size fridge is like $50 on Craigslist!