It's absolutely plausible that we kept tabs on Martial - we do have a huge scouting network after all - but wanted to wait until next season to let him get some experience and to assess him further. However, Van Gaal might have lost confidence in his forward line which is understandable, especially that even before the crappy start in the league (in terms of attacking performances) he said we would need an extra attacking player. So we decided to take action now. Bigger and better targets (Reus, Griezmann) are probably not available. At least I hope that is the case, and not that we haven't even tried...
I'm only guessing, of course, but it seems to make sense to me. The rumoured fee is insane though, no denying that. If he feels he's worth it, go ahead; it's his job that's on the line.