The likelihood is that everything very much is dependant on how much sponsorship Mr Levy is able to bring in, it maybe that the new stadium size will service the debt should there be a shortfall. So as Mr Levy has said many times, building the stadium would not influence the current budgets to a downturn in what we spend currently.
At the moment a lot of the figures are up in the air, current totals that were spoken about originally of 450m were a couple of years ago, one doesn't know how much of this figure has already be found. For example, the club has already purchased over 90% of the land needed for the actually stadium and work has already started on the Supermarket.
What we do know is the recent training complex for Utd raised £120m over 8 years. This should be a starting point for Levy to work from IMO. As has been spoken recently in this thread and on skyscraper, Levy has an exciting package to offer companies wishing to get involved. Firstly the stadium, the training ground and the rumoured name change of White Hart Lane Station whilst other smaller details like the building of the stadium website could be sponsored.
Exciting times... if a bit slow.