The Peachy was originally intended to use a Blender plugin to do its own slicing and g-code translation into the Peachy signal. Now however it simply uses G-Code from any source and translates that to the Peachy signal. Almost any modeler will technically work, but the things to keep in mind:

Any thing you want to print needs to be a solid object in the software with no intersecting or mating faces, (this is IMO just good 3D modeling practice in general, but those kinds of errors cause the slicing/printing to fail.)
You need to be able to export it as a .stl file after getting the object solid, there are slicers that can use other file formats, but .stl is universal.
You need a slicing program. Opinions differ on just which one is best at any given time, and sometimes these things break with new updates, so I recommend learning multiple slicers just in case.
Once the model has been run through the slicer, you get g-code. This can get run through the Peachy software for printing.

Take note: SketchUp is not optimized for solid modeling, Blender is not optimized for mechanical modeling, Meshmixer is not optimized for very complex functions, and Open SCAD is not optimized for use unless you can think in code. There are people who will swear up and down that these programs are each perfectly valid choices for these functions, and it's possible to use pretty much any modeling program for any model... It's also possible to use a claw hammer to cut paper, but we don't. Learn multiple software packages, and use each one for what it does best.