We could have bought (or kept) a fit striker, wouldn't be much good if they aren't very good. The last top forward we bought was Lineker, and that was just before he became a star player (thanks to the season he had with us, and the subsequent WC). So in 29 years we have bought one top striker, which shows just how hard it is to do. You either have to pay big, or get lucky (either by getting a top forward on the cheap who hasn't reached their potential, or someone coming through the youth team). Given that it's very rare you get lucky, that means weave to pay big for a top forward, money we don't have.
You can strengthen any other area of the team without having to pay big money, but not upfront. United won the league last year with a pretty average side but a fantastic attack (ie. Van Persie). Upfront is the difference maker. Ability wise, if we bought James McCarthy's equivalent upfront he wouldn't get 10 league goals a season. To strengthen upfront means we have to go to a higher level than players in other areas of the field. Look at Fletcher, Carrick, Cleverly and the ilk this season without having the 'difference makers' Van Persie, Rooney winning matches for them. The top end of the field is the business end, wins games, requires the most talent; it's by far the hardest to adequately fill. So all this talk of "we just need a striker" completely overlooks the quality required that would actually strengthen the team, as opposed to just putting another body up there. The glass ceiling is upfront. Shane Long would have been the latest in a long line of average joes to play there for us. As an Ireland fan he's more talented than McCarthy, but again when it comes to upfront the level required increases and it's something we haven't done since Lineker.