I will try to give you a practical answer top your question about flow rate. The hot water demand (in gallons per minute) of various fixtures is: bathtub=3.6; Dishwasher= 1.5 (and less if you have a dishwasher with an electric heater option); kitchen sink= 1.6; lav=0.3; shower=2.5; and clothes washer=3.3.
Your setup requires 2.5 + 1.6 + 0.3 +/- 3.3. Depending on whether or not you plan to run your washer with the other fixtures, you will need a unit capable of delivering at least 4.4 gal/min, and perhaps as much as 7.7 gal/min.
As I ststed in an earlier post on this subject, you need to calculate the temperature rise that your unit is capable of (temperature rise is defined as the number of degrees (usually Fahrenheit) difference between the inlet water and outlet water temperatures (at the maximum related flow rate of the water heater). Thus, a heater may be rated for a 70 degree temperature rise (that’s a fairly common specification), which means, for example, that the heater can deliver 120 degree water from a 50 degree input at the unit’s specified maximum flow rate. All of this should be posted on the manufacturer’s specs for the heater.
One thing I must mention is that the manufacturer’s specs assume that the unit will be installed in a well-designed system that allows for good water flow. If you have an old house with a low pressure supply, clogged supply pipes, or blocked fixture outlests, you will still have problems if you merely replace your conventional water heater. Indeed, a well designed plumbing system with a good tank-style water heater should provide adequate hot water, especially for the minimal demand you describe for your system. If it doesn’t, you have further problems that may need more than replacement of your conventional water heater with a tankless one.