If I update my Steam MW3 whit IW5 would I be possible to have both on same computer (in same folder)?

No, you can just copy the modern warfare 3 folder from C:\Program Files (x86)\Steam\steamapps\modern warfare 3 and just use the copied one. Once you install IW5M, Steam matchmaking won't be accessable from that specific installation.

No, you can't have both the games running from a single copy.

What about my rang at steam? Do I need to start over?

Yes. Your start from lvl 1. But you'll also have x4 XP.

It only takes a few hours to hit lvl 80 (8-11 hours for most) and you have 15 custom classes unlocked from the beginning, so it isn't hard to get right back to where you were (except for total numbers of weapon kills and such, but whatever).

OK thank you four everything.