I recently read up quite a bit on FDM robots, and in my opinion it would be a shame to pass up the opportunity to build a robot yourself, and even design one yourself. The cartesian machines are more or less variations on a theme, and most use the same set of standard parts: nema 17 motors, acme screw shaft, linear rods, linear bearings, 608 bearings, and GT2 pulleys and belts.

Check out Misumi's catalog, and you can always roll the dice on shady-Chinese-company knock-offs (Robotdigg). Look up aluminum t extrusions for framing, or use sheet or square tube. Or buy a frame from tens of vendors who sell kits and parts for the Prusa Mendel. I don't like the swing-set Prusa, and I don't like threaded rods. But there's also the fine Makergear M2 for inspiration.

Electronics and software are also commoditized.

Good kits if you don't want to design your own:

Printrbot Simple Metal Rigidbot (I don't know if they are using the best parts) many other quality kits

Interesting robots:

Reprap Wally, designed by Dennis Brown & Nick Seward.

3d printers are one area where it is the Wild West, you can go as deep as you want in one week, and learn almost everything except software details. All the hardware is nearly standardized, and right out there to look at and study.