Need advice in replacing water heater(s)

I have a 3200 square-foot house with 3 1/2 baths. Occupants include my wife and three kids. The house was built in 2000 and has two water heaters that are both 12 years old. One 75 gallon tank in the attic that serves the three full baths and a 50 gallon tank in the garage that serves the 1/2 bath, kitchen and laundry room. The 75 gallon tank in the attic has gone south and I'm assuming the one the garage doesn't have much life left in it either.

If it's feasible, I would like to go replace both water heaters with one tankless water heater that I would put in the attic almost in between the two existing water heaters. It would require about a 25 to 30 foot run to tie into the line in the garage and about an 8 foot run to tie into the line in the attic. The primary gas pipe In the attic is a 2 inch pipe. The feeder gas pipe that supplies the existing water heater in the attic is three quarters of an inch.

I've done a little bit of research on tankless water heaters... I was looking at the Rinnai RL94i or the Rinnai RU98i. Online, these go for $1070 and $1249 respectively. My thinking was to buy it online and have a plumber come out and tie it all in.

My questions are these:

-Is this a recommended set up?
-Will this create an unreasonable lag time to actually get hot water to the appliances that are being fed from the garage in that I would be adding approximately 25 feet of line.
-Would you recommend one of these Rinnai models, or some other unit?
-Is there something else I'm not considering?

Thank you in advance for your insight.