Fair points all, especially that whoever does come in to central midfield is not a simple one for one replacement. If you go for Modric I think you get as good (if not better based on both players form last season) a replacement, but also more consistent in the number of games you could expect them to play. Less so imo for Nasri, who can go hot and cold it seems to me compared to Modric.
I can't comment on your younger players as I really don't know much about them, so maybe there is a surprise there. But from not starting in the first team, to being the central lynchpin is a heck of a step up.
Sneijder I think however is more of an unknown. Whether, or maybe how long it would take to settle into both English football and your system is a question mark. Some of course can settle from the off, especially if they are given time in pre-season, however not guaranteed, and we all know class in Italy does not always translate into English.
The others I do think are more about building out your squad, so will make you better as a squad (or deeper) and as you say gives you a younger core, its not necessarily imo improvement in the first 11. Although I do take your point that Young on the left may give you tactical benefit, the overall benefit I think maybe marginal as even though it allows you to play Nani on the right, that leaves Valencia in the cold. So giveth with one hand but take away something with the other.
But the bottom line I think is if you are only going for one more significant signing and it is as you assume (and I'd agree) a central midfielder it becomes a critical signing imo. Do you go foreign with potentially more talented options, but risks of adaption, or go with prem experience, but be faced with potentially a very high cost.