I have bought lead-free for my first tube of solder paste and didn't have any troubles. As for the stencil, I'd highly recommend using the soda can method, which will not cost too much.

There is no difference of Lead-free and leaded when doing reflow. Leaded has a lower melting point, but if you are using a reflow oven with a temperature profile, you will not really experience any difference.

Leaded solder is easier to work with especially if your soldering iron is not so good. It's kind of like driving with a stick vs automatic transmission.

Most firms go lead-free is because of legal requirements in places like Europe or California.