Engine size recommendation

I say go with the 318. Almost as someone said above, i'd add flowmasters, headers, and just to make some of the Mopar guys here groan.... A ThermoQuad. You'll love the gas mileage and you'll also love it when those secondarys open. :) Keep it cheap and simple till you can afford to blow cash on gas for fun.

P.s. The more you change, the more it will cost you to build.