A 4- layer board will provide the best RF performance since a solid ground plane can be included in the design. The plane should be on the next layer from the surface on which the module is mounted.
A 2 layer board often provides sufficient RF performance for many applications. Thinner boards are usually better for best RF performance. Signals should be routed between the outer pads to get to the inner ones. Most design houses should allow those clearances. Vias inside of the inner pads are fine as well. Bypass capacitors are not specifically required, though having them won't hurt anything. We provide a 4.7uF bulk capacitor on the module itself.
As far as grounding, if possible try to keep the plane on the same side of the board as the module. If that's not possible, create a copper pour to connect as many of the module grounds as possible (with thermals on the pads), and route the remaining to vias to the plane on the other side. Then, use a number of vias to stitch the top and bottom pours. We have two example designs done in Eagle that will give you a better visual than this text. Both designs have Eagle design and PDF files:
These boards are also available on OSHPark: