Agreed, I don't think he has the staff currently in place to take on the project. And the fact that sandbox games are commonplace now does not make them inexpensive at all. These types of games are typically made with teams of 200 people, maybe more.
There are a couple of factors in his favor, though... one is that some (unknown) amount of assets for the next game are already made. Many of the textures for 1 and 2 already have very high resolution versions of them, as we've seen thanks to high res hacks from the fan community. So it's possible that a bit of the work has already been done, and is of acceptable quality for HD systems. On the other hand, though, the texture work in the previous two games was all hand-painted. If it's cost prohibitive to complete the work with hand-painted textures, as it would surely be quicker and cheaper to use photos, then that amount of work that was previously done will be useless.
While Suzuki was with Sega, he had the advantage of infrastructure and staffing and other administrative tasks being handled by other people. Running his own team and his own company both administratively and creatively for a large project is going to be a huge challenge.
