Your arrival will almost certainly be in Terminal 3. Departure will be from International terminal G, which is connected to T3 airside so there's no need to re-clear security - just make sure that you don't accidently exit the secure area or you will need to re-clear security!

Depending on exact gates it'll take 5-15 minutes to walk gate-to-gate.

80 minutes is more than enough, as long as your inbound isn't significantly delayed.

There is a United Club you can access (as *G) just as you enter the international terminal - on your left about 20 metres after you get off the escalator.