The Sinar: I have used a P2 for 15 odd years on location photographing architecture, so: YES it is verry well suited for it.
However, you don't say what model Sinar you are talking about.

The 75mm will be used mostly although you should check out the Image Circle of this lens, I have the Schneider Super Angulon
The 180mm will be used a lot, lot less in architectural photography, but is a fine lens nevertheless.

The 75mm Mamiya is longish for this type of work, you should have a 50mm aswell for interiors.

Value of the Sinar kit depends on the model: a F model is a lot less expensive than a P2 or P3.
For the value of the lenses you can check KEH.

Peter