You should be able to add a rack and fenders to the Seek 0. Of course, that will add to the total cost of the bike.

A few other responses to your questions:

-If you are considering 40 miles round trip, 5 days/week, that is a lot of riding, especially if you aren't doing much (or any) now. There are some regular posters here who do those distances, so it's physically possible. However, if I were to ride that distance, I'd have to allow 3 hours per day for the commute (i.e., 1.5 h there, 1.5 h back), minimum--that may be more time than you can (or care to) spend riding. You might want to ride the route once or twice for fun (i.e., when you don't have to be at the other end at a particular time) to get a sense of how much work it's going to be.

-It's hard to get a really good sense from Google Maps, but it looks like the greater LA area has enough hills that you're probably going to want more than 3 speeds if you're riding 40 miles/day. Ignore this advice if you've ridden the route before and you KNOW that it's flat, even for a bike.

-A fixed-gear bicycle has a single speed, and no freewheel--it doesn't coast at all. This forces you out of your most comfortable pedaling rhythm (or cadence), particularly in hilly areas, so it's more work. Probably not what you want at this stage, although supposedly they're great fun. I might buy one someday if I can ever talk my wife into letting me keep 4 bikes in the shed.