American Standard and Kohler both make any models that fit your requirements. The AS Cadet and K's Cimmaron both come in round or elongated, and regular height or comfort height. Do check the overall measurments though, because there are also several elongated toilets that will fit in that space and I would never choose a round toilet unless I absolutely had no other choice.

I'm not sure if low-flow and dual-flush kits are the same or different, but on the latter I did try to retrofit an original 1961 American Standard toilet with a dual-flush retrofit kit. The kit is similar to a toilet tank repair kit and replaces the toilet flap and handle inside the tank. Anyway didn't work, as the original flap contraption included this large metal piece that looked like it couldn't be removed without lifting the tank off the bowl, and I didn't want to risk taking apart the entire toilet.

If your looking for the savings on your water bill you are looking in the wrong place, the real savings is on your tax bill. In the early 1900's toilets consumed 5gpf however in those days water was plentiful and the sewage was simply discharged into a local river or estuary. In the late 40's or early 50's they began mandating sewage treatment and as the population expanded there was a big concern that many of the sewage treatment plants could not keep pace with demand. On average every man, woman and child will flush a toilet 6 times a day. At 5gpf that equated to 30gal/day per person or on the big picture, 30,000gal of waste to the treatment plant per day for every 1,000 people in your community. With the rate of population increase by the mid 60's the local sewage treatment plants were being overwhelmed until someone came up with the idea that if they cut the flush rate from 5gpf to 3.5gpf they could reduce the waste load generated by toilets by approximately 30%. The alternative was to build additional treatment plants and replace miles of sewer pipes with larger lines at a substantial cost to the taxpayers. By the mid 80's the population had expanded to the point where not only were the sewage treatment plants overwhelmed, the municipal water supply and treatment facilities were also running at peak demand. In some areas such as where I live water is plentiful so we could build additional water & sewer treatment plants at a cost of millions of dollars to the taxpayers, while in some arid regions such as California or the Southwest, they simply do not have the water even if they were to build additional treatment plants. When we consider that toilets account for approximately 50 of the total water and sewer demand the quick solution was to reduce the toilet water and sewer consumption rate to 1.6gpf, thus reducing the load generated by toilets by approximately 50%. Now in regards to why you cannot see the savings on your water bill? As Kudzu pointed out, the national average cost for water is $6.50 per 1,000gal or $0.0065 per gallon, however, water supply is not measured in gallons but rather in cubic feet. One cubic foot of water is slightly less than 8 gallons (7.92gal/cu.ft) therefore on the national average water costs $0.051/gal, however for billing purposes most communities charge a flat monthly service rate that includes the first 5,000cu.ft, then they charge a surcharge for each additional 500cu.ft. Sewer tax is then computed at 2 or 3 times the water rate. On the individual household, in most cases changing from 3.5gpf to 1.6gpf will not change your consumption enough to show a substantial savings on your water bill, on the other hand, if the community as a whole does not make the change, they will have no alternative but to replace the sewer lines and build additional water and sewer treatment plants so in conclusion, if your looking for the savings on your water bill you looking in the wrong place. The real savings is on your tax bill.

diadon- Even if you are in an area where Toto is not widely available, they are easy to order online. They are widely available where I live, but I ordered my most recent one from Amazon since it was the best price, even with shipping. There is nothing exotic about installation: you or a plumber will have no problem. I like them because they are simply a high quality, reliable product. I now have four of them in my house and have never had a clog in 10 years.