I'd be wary of compromising rigidity for weight at all. It's much more important to have a strong machine than a fast machine.
I don't think I have compromised the rigidity really, the machine will be fairly heavy duty, but I've tried to use aluminium components where I can to help keep the weight down. I agree that the speed isn't as critical as a strong structure.

Well, I had one ballscrew that was very bent, of course when I bought them they were all packaged up, so it was hard to see that they could have been bent. I've managed to straighten the worse one almost completely, but am awaiting loan of a friends press to enable me to straighten it further. I'm wondering if I could get the ballscrews under tension whilst on the machine by tightening both ends, and hopefully pulling it straight?