Contraptor

The typical flow for CNC goes like this: make model in 3D or 2D CAD -> generate toolpaths in CAM -> export GCode file -> execute GCode on the firmware. It's difficult to say what are the best choices for each step because each tool has their pros and cons and it depends on the type of things you'd like to cut. Some people prefer Sketchup, others OpenSCAD. For 2D modeling, Inkscape is a good free app. All of these have (with plugins) DXF export, either 3D or 2D. For CAM, you might want to look at free CamBam version. Though rather old, it's is also fairly clean in terms of workflow. It can import 2D DXF (not sure about 3D), and use it as starting point to generate toolpaths. For Arduino UNO you can use Contraptor default firmware (early fork of Mike Ellery's Arduino GCode Interpreter by Chris Meighan), or grbl and its descendants (e.g. Smoothie). grbl branch is probably better because it's optimized to achieve higher step frequency, which is important with AVR-based Arduino, microstepping and leadscrew-driven machine (assuming your CNC has high-ish screw thread per inch/mm count). Each firmware usually (but not always) has a companion host app (or recommendation for one) that can be used to send GCode files from the PC. Contraptor GRemote has been tested (though not thoroughly) with Contraptor firmware and grbl; it does not have a 3D simulator (yet?).