Chaps, I hope these few words may be of assistance, these are issues of a technical, not an economic standpoint. (The deliberate hike in Airbus support fees was a primary reason for services being stopped in 2003).
- No holes were ever drilled in anything, this rumour is rubbish. (And as ponted out, drilling holes in the cabin floor itself would have been pointless). When the aircraft were decomissioned, all fluids were drained, the escape slides removed and the Ground Power Protection Unit was removed from the elctronics rack.
- Yes the draining of the hydraulics was crucial. Concorde used a special mineral based fluid, Chevron M2V, which was highly prone to water contamination and always had to be stored in airtight containers. M2V was required because conventional fluids (eg Skydrol) are useless at very high temperatures. The entire hydraulic system would require prging, and the components over-hauled. (Although Concorde was the only A/C to use M2V, there is an Americal Mil Spec fluid that is a direct equivilant).
- Unfortunately the majority of British A/C were stored outside, which has not helped the structures one bit. (Although G-BOAC & G-BOAE are now safely undercover). And in the case of G-BOAA at East Fortune, although stored in a hangar, the wings were cut off and then re-attached at the museum, efectively killing the aircraft. Many of the French aircraft have been stored undercover, and are in far better external condition.
Economically it would be astronomically difficult to bring an aircraft up to flight standard, but from a technical one, yes it is possible, given sufficient manpower and expertise (no shortage of either). We can all dream.....
I hope this help you guys a little.