Is the zombie scenario similar to this game that's been happening on a couple college campuses recently? I'm in love with zombies so I can't wait to see what you're doing!
The only games I've had experience with modding have been in Morrowind and the Jedi Knight games. I especially like skinning models. Morrowind and Oblivion have huge mod communities because they are so open ended and easy to make large modifications for. The only problem with Morrowind (and Oblivion but not as much) are conflicting plugins. Because the language used for event sequences is so open-ended, it's easy for it to conflict with other scripts. Fun though.
I have an old Wildfire back from the good old days, but the bladder apparently isn't as strong as the RF20. Mine is still in great condition but I think that's the only difference. That and the size. I'd assume ranges would be similar. Better off getting an RF20 at this point unless you're a collector