That unit needs a concentric vent kit. It will not be compatible with your current venting. It uses a 3” flue vent within a 5” outlet air inlet pipe.
Any particular reason for a tankless? Typical situations a standard storage tank system uses less fuel. The primary situations that tankless are a better option are:
space saving requirements
low use of hot water, such as a cottage, vacation house, or only one/two people living in the residence
the need for endless supply of hot water, like filling soaker tubs / spas
If you are an average family for hot water use, in your primary residence, a typical storage tank system is better value overall