the auto 2.5 does not perform that differently from the manual 2.0, on paper any way. I have a manual 2.5 and that returns 25mpg on my daily drive that involves a 7 miles trip into Birmingham each day, however when traffic is really bad, eg the 20 min drive turns into 55 mins, consumption drops to 15mpg, an auto may well be worse. For a significant performance change the 3.0 auto would be worthwhile but they cost much more.
The good news is that the 2.5 is pretty affordable these days, is smooth quiet and despite having that big lump up front, it still handles well for a big car, maybe not as well as the 2.0 but way better than my old Saab 9000. The belt change is more expensive and more of a specialist job than the 2.0, the water pump is best changed at the same time as the original is plastic and the impeller can spin on the axis. The front suspension/tyres get more of a hammering as well due to the weight.
The 2.5 can still come without a CD player so check that and the cable, the rad is just as flimsy, same part, if you have two 166s on the drive you could always transfer any missing bits onto the new car.
and get some chrome polish for those pipes
HTH