Well let me tell ya first hand, 318s arent a bad ride at all.
You wont have a tough time holding 120, maybe 200 yea, these things love to cruize at 140. Yes aswell the gas consumption is pretty good, probably better than the 325, but really is all depends on how you drive your 318.
The 318 has got some balls, where the power lacks compared to the 325 is high end 3rd gear and so forth. On the highway it cruizes well at any speed you give it. but again the acceleration from lets say 140 to 160 will not beat a 325, slightly slower *atleast in my experiances*
I contantly beat my buddy of the line until he hits high 3rd gear, then he inches past me, he drives a 325.
I would say at least consider the 318, its a dying breed!!
take it easy