I don't think the changes need to be that drastic. Quite a lot of deadwood has been shipped out, some need replacing. Two most important signings now are a dominant CB who is good in the air and a creative player who can play across the front (wide, CAM, up front). Baines will also need an alternative and I suspect Koeman will look for a new GK (well, he has actually said this).
Central midfield is finally sorted, which is actually amazing considering where we started last summer (Barry and McCarthy our best combo, now we have Schneiderlin, Gana Gueye and Davies ahead of them). Coleman has improved after a slump and Holgate can replace him if there are injuries. New CB / Williams / Holgate / Funes Mori wouldn't be too bad. I'm still hoping Baines is in a slump but I worry age has caught up with him (those injuries he's had). Lookman will be ready for next season, Barkley would have less pressure to create something with each of his passes if there were more creative players.
So depth chart would look something like this:
GK: X / Joel
RB: Coleman / Holgate
CB: Y / Williams / Holgate / Funes Mori / Galloway
LB: Baines / Z / Galloway
CM: Schneiderlin / Gueye / Davies / McCarthy / Barry
LM/RM: A / Bolasie / Lookman / Mirallas / Lennon
CAM: A / Barkley
FW: Lukaku / A / Calvert-Lewin
Yes, numbers would still be short but every signing does not need to be a starter. I suspect that Mirallas and Lennon might also be shipped out and be replaced by younger players, but not necessary in the 20-30 million bracket. More likely they would be Walsh like signings.