To answer the question of how simple or difficult it is to install DD-WRT: it all depends on the hardware. Some hardware you have to take extra steps beyond just using the original firmware's "update firmware" tool. In fact, there are some hardware where you have to utilize tftp to flash, which can be difficult for a novice.
Once you know the model #, the Router Database at the link Atomic gives above will/should direct you to a spelled out step-by-step guide on flashing your specific hardware, as well as which version of the firmware can be installed. My advice? I would find an older wireless router that DD-WRT's database confirms works with the firmware and practice flashing with it before trying it out on your newer hardware. Read, re-read, and re-read again all instructions before trying.