We had to paint our entire new house, (also aprox. 2500sq.ft.). The builder said he had primered the place, but it was some clay based, chalky like stuff, so I would highly consider a good primer. We went with a local Benjamin Moore dealer, who free of charge, sent an interior designer out and spent all afternoon with the wife. Our main floor has a rather open floor plan, so to try and delineate the rooms, and maintain a smooth transition was no easy task. Our house goes from Victorian, to Colonial, to French country, to white Rattan, with an oak kitchen in between D: Yikes! The designer also helped with wallpaper, tile and window treatments, and had a line on where they could be purchased. The wife is pretty good, but this designers services were invaluable.
As others have said, you get what you pay for. I bought some cheap exterior, *LOWES VALSPAR* enamel for some trim outside, and the stuff was peeling within a year, --- never again!