With a modem /router in one you can connect wirelessly through the house.
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A) It can serve both PC and Laptop - YES and more than two if you want
B) If you can use (and pay for ) just one BBand provider but use two machines on it at the same time - more than likely but will depend on your ISP
C) would you need two routers or one? - One
D) Would you need to "Network" the two PC's? - No