Good Luck Gurvy!
Well it seems your starting where I started a nice steel frame and a nice chunk of aluminum for a gantry. I am nearly completed with the build process and need only to get off my butt and get it done.
My all aluminum gantry is close to 150lbs and is only 40" wide with everything on it except the router. I guess the key is in making the most smooth movement possible as wieght is only relevant to taking off from a stop and slowing to a stop for direction change. Of course the larger the stepper/servo used the less of an issue this is anyway. As to that (motors) i would suggest you calculate how much you need and then double it. So I knew I needed at least 200oz/in and I placed 464oz/in on it instead.
I have zero issue moving this around at 300+ IPM with the router attached not that I would go along anything near this speed while in the wood or plastic anyway it just shows that over built is better IMHO.
Good Luck and have FUN above all else! This is a real addictive hobby/job so be careful you don't make it your mistress! LOL
Coog
Building Stage:[xxxxxxxxxxxxxxxxxxxx] 105%
Finishing Stage:[xxxxxxxxxxxxxxxxxxx-] 95%