We need to spend whatever it takes to get us to the top. Then once that is done, we spend what it takes to stay there.
GK
De Gea
Lindegaard
Amos
We are well set in this position for the foreseeable future, so the key is giving Dave the big contract that will tie him down and probably finding a replacement for our backups. This is likely to be next sumer, as I can't imagine how much longer Lindegaard is going to want to sit on the bench.
DEF
Rafael
Shaw
Rojo
Blackett
Jones
Smalling
Evans
A lot depends on how these players progress this season. Smalling and Jones were touted as future first choice England CBs, and they certainly have promise, but they need to start realising that. They will play consistently at CB this year, which is what they've been crying out for. Hopefully they step up to plate. As it stands currently, we're missing leadership more than anything at the back, so unless one of these lot take on that mantle, we should look to the transfer market. One thing that does stand out is the lack of options at LB and RB, so LvG has to decide whether the cover can be found from our youth team or whether he needs to spend. The other main issue is injuries. This should be the last season where we tolerate consistent injuries. If Rafael, Smalling and Jones once again suffer the same amount of time away from the pitch because of injuries, we should look to replace them with players less injury prone.
MID
Herrera
Anderson
Carrick
Fletcher
Di Maria
Blind
Januzaj
Young
Valencia
Fellaini
Still needs quite a bit of surgery. Herrera, Blind and Di Maria should be our first choice midfield 3, while Januzaj has huge potential so there should be no question marks over those four. Carrick is a good option for this year at least, but we should be looking to replace him next summer at the latest. The rest are dispensable and the sooner we replace them the better.
ATT
RvP
Rooney
Falcao
Wilson
There is no need to look for new strikers in the next 1 or 2 transfer windows, provided they perform as they should be doing and provided we secure Falcao permanently. Apart from Wilson, they are all getting towards the latter stages of their career, so regardless of form, we will soon need to dip back into the market, although that's a problem for summer 2016, not next summer.
The players in Bold are the ones we need to ship off as soon as possible within the next two windows. As you can see, it's only midfield where we definitely need to replace players. The players in Bold and Italics should be replaced depending on certain circumstances. This is mainly in defence, where players need to 1) stay fit and 2) show they can be first choice United defenders. I've think the reserve goalkeepers are in the same boat in that it's mainly up to them - if they are happy, keep them. If not, sell and replace.
So, in answer to the OP, we need to spend big in the next two transfer windows to replace the bolded midfielders. We also need to add more depth at LB and RB. If our Bolded defenders realise their potential, the need to splash out on defenders is reduced. If not, we need to get at least one CB.
January: offload Central midfield deadwood (fellaini) - bring in Strootman
Summer 2015: evaluate performance of defenders before buying/not buying replacements - offload Young, Valencia, Carrick, Fletcher and buy replacements. Buy backup LB and RB.
After all that, we should be looking to add no more than 2-3 players each summer to keep us ticking over, (ala Bayern) and not spending huge to get the shiniest new players (ala Madrid)