I agree with The Infinite Shindig, get out and nerf, even if it is still raining. I got back from a war a few weeks ago, and I loved getting back into the wars (even though it was raining). I think that there is no point in even buying a nerf gun if you aren't even going to nerf. I don't think that there should be any excuses for not nerfing, because all of the problems that you will encounter can easily be solved. I can't wait until school gets out and I can start pumping out darts and having nerf wars constantly.
I am sorry; I don't think you can have rice as ammunition. First, every grain of rice is different, so there would be a horrible seal between the rice and the barrel. You would also have a hard time putting the rice into the barrel. You would have a hard time finding tubing that would have a small enough ID (Inner Diameter) to fit rice. Finally, as you said, there would be the problem of rice falling into the firing mechanism.
It seems like the mod is taken care of, so I'll answer your questions. 1. A good gun to get is an AirTech 2000. You can get 2 of them at target for around $10. They shoot around 80ft from what I have heard (I haven't modified mine yet). The gun has 4 barrels that you have to manually rotate. This is the gun that most newbs get.
Picture of twin pack: 2. No, modification isn't just making the barrel longer. The main part of modification is just taking out/dremeling all the restrictors. After, you usually want to put on a different barrel, so you can have a lower friction barrel that fits your darts better.
PM me if you have any more questions, I would be glad to answer them.