how much dpi am going to use?
How are we to know that?
Also: It’s ppi, not dpi, if you talk about pixel images.
In generel with large prints that are viewed from more than arm’s distance 72ppi effective resolution suffice.
But please read up on resolution in relation to viewing distance.
You should also ask your print provider what effective resolution they deem necessary for the format.
Ask the print shop for their output requirements. Large or ultralarge format printing requires a pretty low DPI, so scanning your photo at the normal 300 DPI and then blowing it up to the final size will probably be enough.