I would say buy the ponchos at WDW. We have brought the dollar store ones, and we have brought more expensive ones, and both times, we wound up pitching them in the trash and buying WDW ones. The WDW ones just seem to work better. Basically, when it rains at WDW, you just throw a poncho on and just keep going. We have done a "monsoon" rain in the winter and we just kept going until we were too cold to do more- you shouldn't have that problem since it is summer. You will want to figure out shoes that won't drive you crazy if they are soaked. You also want to be sure and have some shoes to alternate, because depending on your shoes, they may not dry out by the next morning, plus shoes will rub a little in different spots when wet, so you might need a change. I also highly recommend using body glide on your feet in general to avoid blisters, and when it rains and your shoes might be rubbing in spots different than usual it can be even more of a help. You will probably want to wear shorts instead of jeans or pants. We had to wear jeans when we did it in the winter because it was cold, and the reasons we finally had to shut it down mid-afternoon was because the bottoms of our pants were soaked which made us really cold.
Most of the rides will keep going in the rain. Test Track won't, so if there is a good chance of rain later in the day, you want to try to pick that one up early before the rain starts. Other than that, you just keep on going and enjoy the fact that the lines are usually shorter.
