I highly recommend NOT going with a 55 gallon. I know it's an ok deal (not great, so don't rush it), and that hearing this right now with the good deal and all may be a bummer, but if you really want to have any depth and complexity to your hardscape, 12'' just does not cut it.
I had a 29 gallon with 30" x 12" x 18" dimensions and trying to scape it really just drove me INSANE, you can't fit anything that doesn't go off angle from left to right, try and fit something that goes towards the back or front, forget it. So what I did, was sell it, and buy an ADA 75cm that has dimensions of 29.5" x 18" x 18" (just more depth) and I have to say that the dimensions are near perfected by the added depth, and are obviously designed for a planted tank upon inspection.
Fish don't need to move both front to back and side to side, they can do one of either and be happy without the other, so company's like Perfecto and All-Glass make their tanks very long and high, but not deep, to give the impression of a larger tank. This method of deception is also at a benefit to the most common use of those tanks, and that is, fish-keeping, because fish are very happy with lots of right to left space, and don't require much depth, unless they are very large. Thus their tanks are able to suit most customers (fishkeepers) and still maintain an impression that the tank is actually larger than it is by putting all the volume into length, appearing to be more space for the $$$.
When you look at ADA tanks (or any tank designed with scaping in mind) you see that they were obviously created with AQUASCAPING in mind, not fish keeping. When designed, they worry less about the appearance in size, but the ability to properly create an aquascape. Then instead of taking away ease of use to make the tank seem larger, they then go the extra yard by taking the trims off, killing two or three birds with one stone, effectively making the tank more pleasing to the eye, built with thicker glass and more quality seals, appear larger, and have an overall cleaner, more elegant look.
Furthermore, this does not only apply to ADA tanks, but all trim-less, rimless tanks made with dimensions that consider the possibility of aquascaping, and of course, made of quality glass and seals, and made by quality craftsman who put thought an effort into each tank. I merely used ADA as an example, because of the their good reputation, and the "oh yeah, I know those tanks, thats true" element.
Basically, if your heart is set on that deal, I don't blame you. It's a good price. Although I do recommend re-thinking merely the dimensions of the tank before immediately purchasing it if there is a possibility of you creating a scape with extensive depth and complexity. Be patient, rushing in this hobby can lead to disappointment and such. Ask the seller if you can have some time to think it over, patience is the key. Good luck, and most of all, have fun!
These pieces of advice are 100% my opinion, and may vary depending on who you ask, so the decision in the end is up to you