It's true that we haven't got a bottomless pit of money like a few other clubs do, no arguing with that. I would argue that the club needs to be better with moving players on who aren't working out much sooner, like how we recuperated a huge proportion of the Lukaku money. This then helps us when it comes to funding other transfers.
There could be a scenario where we could be playing a league game on the Saturday, Champions League in midweek and then another league game on the weekend. It really depends on how we progress in all competitions. Ideally, we want to go as far as we can in all of those competitions and be competing for trophies. So, at the very least, getting out of the groups of the Champions League, aiming for a top four spot in the league (hopefully secure or improve on what we did this year) and get as far as we can with the cups. You can't do that with the same 11 week in week out, it does need freshening up. If the opportunity is there with us being able to use 5 subs, then we need to use that to our advantage as much as possible.
Looking at the squad as a whole, we could argue that we need to be looking at virtually every position. Certainly the goalkeeper situation, centre back positions, full backs, wings and central midfield...there's a lot that needs to be done. It's one of those things where the closer you look, the harder the job seems.
It's very true what you say about Grealish being a focal point at Villa. He will have to work very hard when he comes here and it's not a guarantee that he would be a starter in every game as he would at Villa. But, you want that in your squad, with players having to work very hard to prove themselves. I'd love to see "that" back at United, the feeling that it's a huge opportunity for players and that they need to work at it.