The way I see it Levy has probably already earmarked a certain amount of capital to be used for redevelopment. This probably falls way short of what is needed to build the stadium, whilst still being able to offer competitive wages.
If investment comes the, say £50M can be used to built the new stadium, but the money that has already been put away for a rainy day can be used to spend on transfers and loosening the wage budget.
I wouldn't worry about FFP.
This is Dan Levy we are talking about. If he can take from one pot, put it in another pot to get around FFP then it would be an issue.
It wouldn't surprise me if we heard about an investment, worth say 30% of the club, so about £130M, and also something to do with the naming rights for the stadium around the same time.
Any new investor will no doubt buy the club outright, but once the stadium has been built.
The investment helps this process to begin. The stadium can begin, the club have money to invest in players, and once built, the revenue from the larger crowds will mean that the higher wages aren't an issue. Then the complete takeover, with naming rights all in place, can begin. ENIC can walk away with their £400M or whatever.
FFP wont be an issue as the whole time the club is being run as a business, not as a sugar daddy's plaything