You have two problems to deal with. The first is the 15-1/2" rough-in. Standard rough-in for toilets (all brands) is 12". Sometimes floor joist location will force this distance to be changed. Sometimes less than 12", sometimes more. Now, this problem can be overcome with one of the Toto models that use their patented Unifit adapter. This adapter comes in 3 sizes: 10", 12", and 14". These are skirted toilets which conceal these adapters. So, using a 14" adapter would set the toilet reasonably close to the wall. Toto is the only company that makes these adapters. The next problem may not be as easily solved. Elongated bowls in the old "standard" height are sometimes difficult to find. The higher sizes called Comfort Height and the slightly higher yet, ADA are more common. If you will follow the link at the top of this page called "Toilet Review" then "Shopping" and finally "Toto" you will find a listing of most of the Toto toilets that are available along with the specifications for each. Comfort Height and ADA height are approximately the same as a standard dining room chair, so that might give you a frame of reference. The water supply out of the floor will not be a problem, not exactly as specified, but workable. As far as Toilets 101 is concerned, the best I can suggest is just to go through the hundreds of threads under this section of the forum, and then ask questions. Keep in mind, that all low flow toilets are not the same. There are really cheap pieces of junk sold in discount houses that are a total waste of time and money. The name brands of years past do not have the same quality as they did in the past. Toto is the largest manufacturer of toilets in the world and have achieved this mostly by word of mouth and recommendations from plumbers. Early low flow toilets were mostly attempts to use old designs with less water and that of course, didn't work well at all and gave low flow a bad reputation. Toto started differently by redesigning. Where old line companies tried fancy flushing systems, Toto uses basic innards that are available everywhere.