I'm using the program in Japanese (I can read it at an extremely basic level), but it shouldn't be too difficult to figure it out as you're essentially clicking on each option and seeing how the character changes. There's also demo versions you can download as well.
Each volume on DLSite is essentially just a different base body. Of the four that I bought, vol. 8 had the most diversity in my opinion, but you may want another (vol. 9) if you want specifically characters with a large chest, etc.
One thing I would recommend is to find the config.ini file (in the same folder as your .exe file) and increase the accessory_max number. Some of them have it really low by default, making it so you can't add many things at once. (Of course the more things you add, the longer the program takes to load).