Pulleys are the way forward, they give you flexibility in the final machine and generally make the design simpler. It's slightly more expensive to use pulleys rather than a direct connection and there are more moving parts which means more points of failure but I think that's a small price to pay.

One thing I would say about your design is start pricing it up. I'm guessing those corner connection blocks are the 90x90 ones - they are quite expensive if you need a load of them and they don't result in a machine that is all that stiff (they are great for display stands, not so great for machines). Once you start getting into the nitty gritty of placing components and pricing up I think you'll begin to consider using aluminium plate and box section more.

Looking good though.