I recommend an old/used rainsuit, you might be able to find one at goodwill or on craigslist. An even better place to go is REI during their annual "garage sale" of returned gear. When I lived in Texas, that was a camp-out-all-night party, followed by all kinds of stuff for pennies on the dollar.
The reason I give this advice is not just to save money--whatever you get, it's going to get trashed by your chain, passing cars, road grime, whatever. Nice/expensive goretex or breathable tent-type fabrics are going to be ruined, and ultimately you're going to get wet anyway.
I have a bigger problem with freezing, wet feet when I ride in the winter, and I'm not sure what the solution to that is other than being a badass.
It's not so much a matter of keeping yourself dry in the rain or snow, it's key to have something dry to change into when you get there. Waterproof panniers or a day pack with a rain cover will probably do more for your lasting health and comfort than a great suit. (The REI Traverse 30 pack from a couple years ago is an example of a nice daypack with a separate rain shield in a little pouch.)
Hot weather's pretty straightforward, I just wear street clothes or basketball shorts, whatever. Spandex looks pretty silly to me unless you're in a race. A helmet with a visor helps, but I'm sure you have one already. Trek makes surprisingly cheap and good helmets, compared to everything else in their catalog which looks pretty pricey to me.