OK, it's done. I have installed the tankless (several weeks ago) and have used it long enough to report back to everyone.
I have a Rinnai 53i and this unit just fit into the tight vertical clearance in my crawlspace. I got this specific model because of it's low vertical requirement. Space consideration was the #1 reason for switching to the tankless.
Propane company did the gas part of the install. I did the plumbing. Very straight forward, especially since I redid all the plumbing in the house as part of a bath remodel.
The 30-50 water pressure fluctuation from my well pump/tank system does not seem to make any difference. It works fine. We experience about a 10-15 second additional wait for hot water, and this is acceptable. Flow rate is fine (but the incoming water temp is warmer this time of year). We don't like the cold water sandwich and don't like it when the hot cuts off when the flow is too low. We hand wash dishes. I may install a small electric "buffer" tank downstream of the Rinnai which should solve those issues.
By the way, does anyone know what the hit may be in flow rate/pressure if I install a sediment filter (just after the well tank)? I fear sediment buildup in the heater since it may be prone to plugging up. I installed a 10" cartridge housing but have not yet put the filter into it yet. If I use a 30 micron filter will it block much flow? By the way our water is crystal clear and no sediment is showing in the clear empty housing. Might this mean that I don't even need a filter, or is this false logic and small particles are flowing through?