I don't think he wants to use a digital back lah!
6x4.5 cm:
Mamiya 645 are easy to get cheaply, they have interchangeable backs if you shoot with different films often, and lenses are easy to get used in SG. If you are no tparicular about the model, get the M645 Super wihich still has changeable backs, so you can use a polaroid back if necessary. If you don't need all this, you can look for the older M645 1000S (stronger metal body), bu tno internchangeable backs. Lenses will fit all models from the M645 to the latest M645 AF-D......
Other options are:
Pentax 645, P645N P645NII (latter two are AF cameras), lenses are hard to come by in SG
Contax AF645: can get some used ones on Clubsnap.... but expensive
Bronica Etrsi, Etrs (lenses easy available)
Fuji 645 rangefinders
6x6 cm:
Rollei(cord) twin reflexes: varies from SGD 300->2000
Rolleicord copies like: minoltaflex, too many to recall.......check the used camera shops....especially Camix, Prime and Camera Workshop....
Bronica
6x7 cm:
Mamiya 7II (rangefinder though)
Pentax 6x7 (get older ones, good enough), bu heavy body and loud! Lenses are cheap and lightweight...
Mamiya RB/RZ 67
Bronica GS-1
6x8 cm:
Fuji GX 680 (hard to find in SG)