The Hardware of a RepRap machine is perhaps better described as the Mechanics or the Mechanism that is the Machine. So we are thinking here of the rods, motors, belts lead-screws, box, frame, nuts & bolts etc. Everything that the Motors or actuators animate and the electronics drive. The bit that squirts molten plastic known as the extruder is definitely included in this.
As ever (diversity is a recurring theme in the project) there is a bewildering variety of ways people have put their hardware together to do the same task. ie print out a 3D object in plastic. The most common format though is that the mechanism or bot is usually a Cartesian Machine (has linear X,Y & Z axes) driven by stepper motors. This type of mechanism lends itself well to following the Cartesian type commands that are sent to it in G-Code. I guess seeing how the numeric XYZ G-Code commands translate to XYZ physical steps is very intuitive.
Having said the above there is experimentation going on with an amazing range of Bot formats ie Articulated arms, Delta Bots, Turntable, Polar Mechanisms etc etc etc.
If you just want to get a machine up and running and the physical mechanical thing is more yours than the virtual (software, firmware) or electronic creating your own Cartesian Bot is the best way forwards as you can readily use or adapt existing firmware, electronics and software to make your own machine.
Ultimately if experimentation with printing objects is more for you than the bit that does it, and you just want to buy a machine with all the rest thought out (and reasonably expected to play well together) then the Kit variants Makerbot and the Bits from Bytes Darwin machines are going to be your best option.
For components it is always worth networking with other makers as bulk purchases are often cheaper (even if only a saving in carriage) and someone may have surplus items to sell. Ebay is also worth a look search for RepRap.
