Just had another think about this, and it occurred to me that I forgot to ask one important question - how accurate do you want these 25mm holes to be? I said that my big ali gantry wouldn't be stiff enough, but I was judging that by standards for precision machining of stuff. I did the same calculations (using routercnc's excellent spreadsheet that's lurking somewhere on this forum) with a piece of 200 x 100 x 5mm steel box section and get a tool deflection of roughly 12 microns, assuming the centre of the drill is 150mm from the beam - would that be good enough for your purposes? FYI that beam would weigh something like 28Kg.