If you learn by doing, then building one based on a RepRap design is a good way to go. Try to steer clear of the ready assembled ones. They are mostly very good, but you will not satisfy your curiosity.

My experience has been with the MakerFarm Prusa i3 8". (Makerfarm makes the kit. Prusa is the guy who designed a 3D printer in this format. i3 means that it is the third iteration of the design, and 8" means that the print bed is 8 x 8 x 8.

The kit comes with everything except a power supply, a piece of glass to cover the heated bed, and filament (3mm), so you won't be wasting time on hunting expeditions to get the parts you need.

Assembling the kit will give you great insight into how these printers function. Also, users have come up with some useful add-on parts that you can print, as well as some mods that improve the ease of use.

By all means, don't take my word as the last word. Explore all types of printers that come in kit form, but as I said, put ready made ones low on your list of preferences.

Old man Emu