Sarri as a coach isn't a gamble. He's a good coach. The problem with Sarri is the squad doesn't fit to his tactics. Even if he decided to return to 4-3-1-2 from his Empoli days you would still need to make many changes. It would also be a complete u-turn from the way we play now. It would take time for players to adjust.
It wouldn't really be as drastic change in terms of formation as much as it'd be in switching from counter-attack to possession.
Mancini in Italy plays possession football, we have guys like Barella, Sensi(?), Bastoni and even D'Ambrosio & Gagliardini who were part of that. And these two also were here with Spalletti [though they probabyl won't survive the summer].
Guys like De Vrij, Skriniar, Eriksen, Brozovic(?),Sanchez etc thrive on possession football.
The set up, if we opt for Bastoni as left back, won't be that much different. Hakimi will still have the entire flank and the only concern is that we won't have someone on the left capable of controlling the entire flank but with say a forward that drifts out wide (Mertens style), and any of Eriksen/Sensi/Vidal helping out on the left, it won't be much of an issue offensively.
I'm still not a huge fan of the 3-5-2 with our current roster, but if we want to keep this, then Inzaghi knows how to enhance it.
But if we opt for Sarri or Allegri, I'm expecting us to finally switch to 4-3-1-2, which is the system that suits our team the most. Not that many transfers will be needed, even with the +80m demands for cash.