For Simple Bots, the ideal solder is .032" diameter 60/40 rosin core solder.
Feel free to try slightly thinner or thicker diameter solder, but it is highly recommended that you stick with 60/40 rosin core solder.
It is fair to assume the solder contains lead, unless stated otherwise on the packaging.
Working with lead requires precautionary measures such as:
Always wash your hands with soap and water after handling solder to wash the lead off.
Take common-sense precautions like not wiping your eyes, nose or mouth, to avoid absorbing lead.