Imo, and I know a lot will disagree, a successful window is when you sign the players you are interested in. Whether they turn out as good as you thought is down to the player and possibly the manager. When we signed Falcao for instance I bet a handful of people predicted he would be the biggest flop of the season (in world football?). I wonder what odds we would have got. Same with Di Maria, who was one of Madrids best players. Who would've thought he was going to struggle here? You can't blame the club for that as everyone at the time had big expectations of the two.
Fergie kept going on about there being no value in the market and he didn't like the idea of spending absurd amounts of money on one player. Yes Rio, Rooney, Veron, Berba in their day we're expensive but I don't think Di Maria is a Fergie type signing. ~60million is a hell of a lot of money for a footballer but that's the way it is now. Times have changed. Maybe we should have bid more, paid the agent fee or higher wages to those such as Hazard, Lucas, Sneijder, Vidal etc the list goes on.
Now under LVG and Woodward I think we will have more successful transfer windows in terms of getting targets. Whether they turn out as well as hoped is another story but we are not holding back the money, which has always been there.
I used to detest City, Madrid, Chelsea, PSG for their transfer policy but it seems it's the only way to progress in world football these days. Even Barca who are renowned for their homegrown players spent many many millions on Neymar and Suarez.