Another +1 to the above. I commute with a single-speed in good weather. On rainy days, I happen to have another bike with fenders, that's a 3-speed.

With single speed, there's no drivetrain to worry about, so you can avoid a lot of the issues that normally come with lower end bikes.

I don't know your wrench skills, but in my opinion a lower end bike comes with a bit of up-front maintenance work that has to be done, basically to make sure it was manufactured right. This includes making sure the bearings are lubricated and adjusted correctly, and the spokes sufficiently tensioned. Also, depending on your skills, a single-speed conversion is a great use for an older bike if the drivetrain is trashed, and the effort is pretty much the same -- bearings and spokes. Once you've taken care of those things, virtually any bike can last a long time.

Make sure you get the form of bike that you prefer, because you'll be riding it a lot. Most notably drop versus upright handlebars. It's not hard to switch from one to the other, but there's some cost involved due to controls and cabling.

A crank with standard bolted chainrings is a plus, in case it takes you a couple tries to dial in your preferred gear ratio. Also, clearance for wider tires is worth looking for -- at least knowing up front what the bike can take.