I would simply mimic the style of your existing house. It's really just a box when you think about it- you can make it as fancy or plain as you want. I'd be inclined to make it really fancy or unusual, but I'm strange that way! Depending on your climate, you may need to put some kind of footing in the ground to get below the frost line in order to keep it above freezing without too much trouble. Here in the south, termite prevention would be an issue, too.

I built mine, it is just big enough to enclose the well head and the pressure tank. Same construction as a house....a mini-slab (you can mix the concrete with bags), use J-bolts, PT lumber for the bottom 2X4s, studs, batt insulation, electrical, light (in and out), extra plugs on the outside,door, slanted corregated metal roof (slanted). Exterior is your choice, I used siding material and trim on two sides that are not visible, and plywood covered with native flagstone on the visible sides. Oh yes, a hose bib on the outside. The siding material and door have deteriorated after 27 years, but the rock and the roof are still going strong.