lolo, here is something I've thinking of exporting for the past 13 years.
size to price was a major issue at the time.
Different makers but I haven't heard of takagi or rannai yet ... however, rinnai is a pretty major brand for heaters, stoves, and tankless units.
I've been on the tankless units for the past 15 years ... no choice in the matter actually.
There are various sizes and can do heating oil, LPG or natural gas. I've been on a LPG for the past 10 years.
There are even ones that will recirculate your bath water to keep it nice and warm for whenever you want to take the next bath (warning ... Japanese don't use soap/bubbles in the bath tub)
My crappy unit holds up really well for oodles of showers in a row.
Problem with my dinky unit is that 2 showers just won't cut it. Also, flushing is shocker. However, the temps are now electronically set and you don't really need to adjust those dam facuets to get the right temperature for your shower.
You do need a ventalition duct to run one in the house ... electric I'm not sure.
The nice thing about these things is that hot water is on demand. Versus a tank which goes cold and needs time to reheat the water.
Lag time from unit to faucet is not really that bad. Of course, shorter the distance the faster you heat up.
PVC probably won't work ... at least at the top of the unit. However, from there on out ... the heat is not that bad where it will melt anything. The blow off does sort of heat the area where the tank is located so you can dry towels pretty quick.
Don't need steel. You can get away with alloys possibly even aluminum.
Older models were located right over the kitchen sink with there own tap. Sort of weird in the beginning but after a while they grow on you. I've always hated being the last one in line for a tank shower. I always got screwed
Now, of course, all this information is regarding to units I have used in Japan.
Anybody interested? I'm sure I can start digging around and possibly get em over to the states. I'm not sure what the difference is between US and Japanese natural gas is though ...