derew,
Any 3d Printer is fairly 'hackable'.
If it uses an open source design, and no custom designed parts you shouldn't have an issue.
Cartesian designs are probably the easiest for a pure beginner.
CoreXY offers an alternative means of motion and is a little bit more complicated to understand.
Delta printers can be the most fickle to tinker with.
My personal recommendation? Go with the style of design that interests you the most. My first printer was a CubeX closed source printer I ultimately converted to open source which turned out to be incredibly simple after spending a few days reading the reprap wiki and the forums here.
-obelisk