I know the maker and I sent your questions via MSN to him just now. Here are the replies:
@Pop
well seems like UE needs to the GTA: SA Folder set...
Click "Tools"-->"Options" set the GTASA Path to the GTA-SA.exe folder (where the shortcut for GTASA is pointing)
But to answer the main question.. you cant, weapons get added threw the script itself, and that handles weapon strength, and effects
@gangster 101
well , yes/no yes you can edit the the wheel size as far as the game sees it, but the modles wheels might not fit (need to change the modle size with Zmod2) and the paint jobs is on the "Model" tab (I think) but where it allows you to change the dff/txd files, there should be a tab for paintjobs