Regarding biomes and details. I think if you have amount of details that can fill 48x32 inches and it not gonna look like empty parking lots, it gonna cost you something like 1500$+ with rights you want for a map.

My suggestion about resolution will be 7200x5400 or 24x18 inches if printed at 300 PPI. I'm interested in working with you, Zeyami. If you are interested in more discussion and working together write me at vdm.bulgakov@gmail.com