Is it advisable to use lead-free PCB and solder paste on a first SMD assembly job?
While going green is a laudable goal, the real reason most firms go led free is because of legal requirements in places like Europe or California (USA).

If reliability is more important than green then why not stick with leaded solder, assuming of course that you are not requires by your customers to provide lead free or ROHS compliant product.

With lead solder you do not need to be concerned with "tin whiskers" which can cause your boards to fail in service. For more information see: http://nepp.nasa.gov/whisker/background/

As for a stencil, they are generally worthwhile.