Looking to do my first conversion (looking at a Truggy)

[Happywing]

Welcome!
You're on the right track. The ERevo is nice (I have one) but it's nowhere near as durable as most 1/8 truggies. It won't handle nearly as well either.
First, you have to pick your truggy. Truggy rollers can be had for a good price these days.
There are kits for the 8ight-T and MBX5T at this site:
http://www.rcproductdesigns.com/
These kits come with all the hardware you need and instructions for assembly. They use the existing holes in the chassis, so no drilling.
The popular choice for ESC/motor on 4s is probably this one:
http://www3.towerhobbies.com/cgi-bin...?&I=LXVHA5&P=7
Using the batteries that you have will be risky. 25c rating is nice, but they don't have the capacity to handle the motor draw. You would be better off getting a nice 4s pack in the 5000mah range and a 20C minimum rating. Prices for those are coming down.
Here's a shot of my MBX5T with the new RC Product Designs integrated battery tray: