if the brake track of the rim looks like this: )(, rather than like this ||, you should replace the rim, as the brake track isn't very thin, and when they wear can behave in a rather unpredictable fashion and fail pretty spectacularly when the tire is inflated. 15,000 miles sounds like a lot, but it's probably ok.

as far as spokes go, thicker doesn't always mean better. if you want the best possible wheel build, go with double butted spokes. i'd go into more detail, but i'm too tired right now. i'm sure someone else will come along to explain soon enough.