Okay. I need to weigh in on this for 2 reasons:
1. Some of this is a little misguided/Out-of-date
2. I helped my mom sell houses and help people make this decision a lot.
Gas tankless does not require a second vent line. It IS prudent to have one, but not always up to code. Since your are using natural gas, the vent system is already built into line, so making another is redundant. Check your local building codes and with your local Fire Marshall, because they are the best resources. It’s all up to what state you are in. In the Midwest, you may need to do this. In Texas, nope.
Water softener? Again, not necessary, in most cases. Water softeners are really only needed in super rural areas and areas that use well water. That is a completely unnecessary expense, especially since most municipal water supplies ADD softeners to water in the first place. Again, it’s down to where you live. You may have to do this is the Midwest, but in Texas, nope.
Upgrading breakers and wiring? Again, not necessarily. It depends on how old your home is. If your home is less than 8-10 years old, you don’t really need to touch anything. Older than 15? Then, you’ll probably need the upgrade. This one comes down to age, and how much of a power vampire you are.