I think you need to determine exactly what processes you'll use to do the work and the horsepower you'll need for those processes. If you're going to be drilling large holes you need lots of horsepower to be productive. Same thing goes for driving a large tap, which I assume you'll be using considering they're pipe threads. The drill and tap manufacturers can help you determine the horsepower.
Moparfever, thanks for the advice. Actually the volume to start up is not very high and the machine shop that is currently doing the work, drills the flanges and then hand taps them while they are still on the mill. Usually 2 to 4 flanges per job. That is the primary reason for wanting to look at a mill, because of other jobs not to machine tap the flanges. Hope that sort of explains it. Thanks