i used to run mine on kit setup with a 13.5t and was fine on astro, biggest thing is the tyres, either schuey minispikes, or dboots multibytes (possibly nanobytes too) or kourse kings are great if your on a budget as they dont wear out too quick.
13.5 motor will be good too get used to driving the truck and will let you have a safe stable drive, if you decide to upgrade dont go much higher the 8.5t and alot of people like 10.5t in a 2wd sct