Milan will appeal, but I really cannot see how a serious judicial body will grant such an appeal and overthrow UEFA's decision based on what has been going on at that club.
First you have the 180 million euro loan that Li needed in order to purchase his majority stake, which he is struggling to pay back. He has had to refinance this loan in order to stand a chance, and it doesn't look like he will be able to pay it back by October 2018 which was the agreement.
Then, in a situation like this, they go out on a massive spending spree and splash around 200 million euros on players. From what I understand, they are far from breaking even, which is a direct breach of one of the most essential of FFP statutes.
Ok, sure, perhaps they will be able to present a sustainable / profitable business plan for the future in their appeal (for instance if a takeover happens there very shortly), but the judicial body cannot only look at "future facts" which are hypothetical in nature - regardless of how impressive a business plan may appear on first sight. Some emphasis must surely be put on such a plan (and this would be to Milan's benefit), but past and current day facts also must carry a lot of weight.
It will be interesting to see what happens, though, as I am never fully confident in the quality (and integrity) of those in power in sports tribunals.