Which figures are you thinking or is the customer requesting?
Gumpaste dries faster than fondant, so I typically use some precolored fondant (like Satin Ice) and add it to the gumpaste. I always model extra just in case something breaks. Use supports where you can (spaghetti or toothpicks even, be careful about warning your client if you use toothpicks. It would not be unheard of for someone to grab a figure and bite into it!) You can also use or make chocolate plastique (which I think is also called candy clay - it’s a mix of chocolate and corn syrup) - the advantage there is that it sets up quickly in the fridge, but also is susceptible to softening in heat. If you make it yourself from white chocolate, use candy colors to tint. The food color markers are helpful for adding tiny details like eyes, buttons, stripes….
Before you model it, think about where the figures will go and how deep your angles are on the cake so your proportions are ok; otherwise you risk a too small or too large figure on the cake. For a first time, it helps to let the figures lean against the side of the cake for support
For supplies, I buy from Pfeil and Holing in NY (http://www.cakedeco.com) and from Albert Uster Imports (http://www.auiswiss.com); for specialized cutters, veiners and modeling things, I buy from Global Sugar Art, Avalon Deco, Squires Kitchen in the UK ...
Start way sooner than you think you need to