Thanks for the info guys!
Just wanted to give some more info on the use of the machine and the design so far.

Im hoping to cut mainly large ply / MDF sheets on it so wanted it to accommodate the maximum capacity sheet (standard size: 8ft x 4ft).
i didnt want to build a router to this overall sheet size as i it will be a bit of a beast and also the cost comes into play. my thoughts were to make it accept the width of the sheet (4ft / 1220mm + a little) and then the length of the board could be machined in two halves by either feeding the material through of turning it through 180 degrees.

Does the length of the gantry pose a problem for general machining and would it be able to handle cutting aluminium? As long as the strength is there will it be ok and will the precision still be to a fairly high standard. If cutting aluminium or harder woods is
Will be looking at getting the water cooled spindle so im guessing the weight of the gantry and Z axis will be a fair bit so the gantry will have to be fairly substantial.

The bed is going to be in a fixed position so this can be bulked up with diagonals for strength and like suggested heavier gauge extrusion, just difficult to balance the desired design with the cost of parts.

The support in the Y direction behind the gantry was to mount the motor in the centre with pulleys to the X ballscrews, so didnt really want to add a support the other end as this was to allow material to be fed in / out but if i can position it in a way that the material can be slid under the support i may add this in.