yer 3.0 mv6.... on a good run of 15-20 miles I get 28-30 mpg provided there arent too many roundabouts. I get 20-23 mpg round town. I think with the kind of driving you will be doing you will only notice 2-3 mpg difference for getting the 2.5
at any rate be very careful with the car you buy. check for oil leaks around the rocker covers. you can tell by oil dripping onto the exhaust manifold. If the engine has been jet/steam cleaned be very wary. pull out the middle plug leads from each bank to check for oil. get the seller to replace gaskets before sale if there is any sign of oil int he plug wells. this is the same for any omega v6.
also take a torch and shine it onto the belly pan. if it's got water on it it has likely got a cracked heater bypass valve which will use up some water and may eventually result in a nackered dispack.
I didn't check these things....

the other major thing to check is for mayonaise under the oil filler cap as that would denote a possible failed oil cooler (a tad pricey to fix!)
These are I believe the most common engine problems to be wary of on the v6 marks.