Hi Kjetor
As no one has contributed i would recommend that you have a look @ some of the build logs for machines of a similar size to the one you intend to make, if you can run to the extra cost you are better served by staying well clear of the round support rail especially as you would like to be able to machine ally, also for a ridgid machine steel might have been a better and cheaper option for the main frame.
You will also require two ballscrews for the gantry to prevent racking. all this information is readily available on the forum, so i would recommend you forget about the design for the moment until you have done a little more research.
Regards
Mike