If you have a rough measurement from the studs to the drain of 12 1/2", you probably don't have enough room to add 3/4 to the wall depth if you use your current toilet. However, you can do a couple of things to get around this.
You can purchase another toilet with a 10" rough though several name manufactures, but the costs are upwards of $250-300.
OR, you can be a little creative. The least expensive set-up like this that I know of uses a common American Standard base with a standard 12" rough, and then you put that with and American Standard space-saver tank (which I THINK is called the Cadet) which is taller, but less deep from bowl to wall. The thinner tank is what will save you the space you need between the back of the toilet and the wall. The whole outfit (tank and bowl) will run you about $120 at the Home Depot.
Just out of curiosity, why is the shower drain causing you to fir out the wall on 3/4"?