Often times newbies want to do the least with the waste and vents, considering many ways of wet venting without the know how and rules. You can always vent the fixtures in the normal fashion, the cost is almost the same, and tends to be easier to come up with a workable layout.
Normally a 4" line is need after the forth toilet enters the line.
A gas water heater can't be in a bathroom or bedroom.
I believe you can get by with an electric heater.
The 30" is mainly about room for your arms.