>>5381506Welcome to customs!
An electric dremel is a very good investment. Cordless are good for ease of moving about, but a corded will give you longer working times with a consistent battery flow (changing/recharging batteries is the cordless's one draw-back).
As for figures to look out for, depends on the custom you want to make. Ebay has sellers who sell off bundles of cannibalized figma/revoltech joints for decent bulk prices. Make sure you don't buy from sellers in Hong Kong or any part of China, and especially if they have a record of neutral/negative feedback dealing with counterfeit or fake items (avoid bootleg joints, they're brittle as fuck). Hobby Base joints can be had on Ebay for roughly $12 a pop (shipped to CONUS and arrives in a week or so), but
hlj.com has sets for a few bucks cheaper ($9 shipped with SAL, I think). The bulk sets on Ebay are said to be a good deal with the larger sets as there's always a few dud-joints in the mix. Revoltech joints (as in produced in 6-packs by Kaiyodo) can also be had on Ebay and hlj where you can select the exact color, size, and type (single-joint or double-jointed).
As for polymer clay, it's okay if you're sculpting from the ground up with it, but not if you're adding on to an existing figure, as the heat-curing process will melt the figure (or deform it if you boil the clay instead). If you're adding ON TO a figure, look into epoxy putty such as Green Stuff.
For your first time, start with simple mods using cheap figures/dolls from the dollar stores. Learning how to cut limbs off and then re-attach using Hobby Base/Revoltech/Figma joints is a good way to practice, along with modding action figures with a limited range of knee/elbow articulation to bend further, are good ways to build experience on a budget.
What sort of figures are you into? Space-opera, superhero, GI Joes/Soldiers, etc. ?
Remember that you'll be making plenty of mistakes, but learning from them is part of the process. Have fun!