Plumbing supplier here. Wife and I had an extensive remodel/addition done to our home last year and a tankless was a no-brainer. Especially after renting a condo during the construction that had a single 40-gallon electric water heater and we have a three-year old. Plenty of cold showers during that 10-month period. Fug that poo.
What HotSauce and Anybodyhome said above is good advice for sure. I'm fortunate to have LP gas already available in the mechanical room where the tankless is installed. I had a Rinnai RU98i installed, which is their ultra-efficient model that requires a condensate drain, which I also already had in the mechanical room.
Make absolute sure you have enough gas flow available where you install, it could require a new larger line be installed or be as simple and painless as a new regulator.
Hardness of your water is a real concern, and if you have hard water you must be sure to frequently do the vinegar flush to remove calcium deposits. A flush kit will run you around $150. I've got very soft water at home, so a yearly flush should do me well.
I would recommend a hot water circulation system if quick hot water is important to you. It takes mine about 30 seconds to get hot water in my master shower about 20' away from the unit.