Rogerika,
1. Personally, I'm a fan of the do-not-mask-when-you-paint-roadwheels set. First, spray the entire roadwheel flat black. Next, take your base color and spray the wheel portion as if you were trying to make it polka-dotted. Overlap the dots, and let just a little black show through...it should be subtle; this give it a nice weathered/faded appearance already. Apply a black wash around the edge of the wheel when dry, and any overspray (which should be minor if you use a small setting on your airbrush) will be taken care of. Easy.
2.I built the Academy Merkava II several (many) years ago, and I think there is an aftermarket photoetch shot trap available. I personally don't like what I've seen offered. If you go that way, directions should be available for attachment. Since I put mine together before the PE was available, I took small jewelry chain, and cut it 4-5 links long. Used superglue and attached #15 shot to one end (This shot size is commonly called "snake shot" and is available in handgun calibers. Made by CCI, I believe, in packs of 10 or 12). The reference book I used showed a total of 40-45 of the ball and chain assemblies. Next, I threaded the setups onto some small brass wire that I bent to the shape of the turret basket. Drilled some anchor holes in the turret just under where the basket attaches, and used some superglue to anchor the wire and shot trap setups to the anchor holes and the turret basket. Hope this helps you.
Gip Winecoff