I am working on a similar sized machine and have been through the same questions on ball screws - seemed to me there was a lot of different opinions floating around. I too decided that spinning ballnuts was a bit out of my league! I went for 2005 in the end, the larger dia will hopefully help with whipping and I am going to gear it 2:1 so as to get faster feed rates, with the option of gearing 1:1 for high res work...

I am coming round to the idea of using steel (ill have to learn to weld tho!) for the frame and x axis but Ali extrusion does seem the best option for the gantry. The L shape arrangement used in a few builds allows you to keep the gantry squat and rigid. Why does your gantry need to be so tall and thin...?

If you need any help with sketchup just ask I have been using it for years and have just about got through the pain in the ass stage !