Custom 1/5 Brushless

[kladal]

Hello. I have a custom aluminum buggy that was originally designed from an E-Maxx.

Here are the specs:
-Motor = 2217 2y 640kv CC/Neu
-ESC = Megapower 120A
-Battery = 10s (two 5s)
-Tires = Thinking of IMEX Swamp Kong

My questions:
1. How should I gear this setup?
2. The current transmission is a two-speed. Can anyone recommend a single-speed transmission?
3. How do I determine what the differential ratios are currently?
4. Would IMEX's Swamp Kong tires be too big (I do NOT plan on much road/even terrain)?

Any help is greatly appreciated!

Thank you.