We just installed a tankless in our summer cottage (gutted and rebuilt whole place this winter/spring using my now vast Festool collection). It previously had a 50 gallon AOSmith electric tank unit. Space is a premium so I wanted a tankless to free up the space that was the closet that held the existing unit. Cottage is 700sq/ft, so we needed the space. I considered Rinnai and Rheem.
Upon talking with friends/family in the Plumbing trade one friend convinced me to look at a brand called Eternal. It is a hybrid unit that is tankless technology, but also contains a 2 gallon reserve tank so you you do not have that ramp up time when you turn on a faucet calling for hot water as the 2 gal tank is always on call. I never have previously used a tankless in any of my homes and was skeptical. I installed the unit in April, running all new gas lines as no gas existed in the cottage. Gas consisted of a LP above ground tank and the Eternal was converted to LP from Natural Gas. I believe there are 3 or 4 units in their line and I chose a middle unit -the Eternal GU145. I do not have paperwork in front of me, but I believe unit was $1,600. Labor would probably have been at least that same cost if not up to $3,000, however I ran all the gas lines and installed the unit, bringing cold water supply to the unit using PEX...Uponor PEX not Depot or Lowes type. Venting consists of a PVC intake and a separate PVC exhaust venting to exterior (vertical or horizontal). Installing the entire unit, supply lines, venting, condensation drainage and gas lines took a full day and was easy. I am not a plumber, but very comfortable doing the work and of course had friend double check everything and pull necessary permits for gas, etc.
This is all with my unit being moved to a small framed out storage nook in the loft are of the attic. It sounds like your conversion would be much easier as everything exists to be tied into.
After 3 months I can honestly say I will never put a conventional water heater in again in any home. A previous concern many bring up is noise. This thing is very quiet. It does come on periodically for a few seconds to keep the reserve tank hot, but it is quiet and does not vibrate. The unit is stainless steel, so rusting should not be an issue (cottage close to ocean). My unit supplies a full bath, a half bath, an outdoor shower 8), and sink/DW in kitchen and unit is more than enough. Showers are endless hot water, no "cold water sandwiching", etc. I love the unit so far (almost 4 months) and it dropped electric bill by about 50%.
This is the only brand I have ever used, but I am very happy with it. Sorry for long reply...
