Need help replacing tank gas water heater with tankless

I have an old gas tank heater in a small closet which is about 3'x3'x8'high and used only for a heater. It is vented straight up through the center of the ceiling out to the roof. I need to know is there tankless water heaters I can buy which can use the hole already there going through the ceiling center and through the roof. I would like to spend up to $350 on a natural gas tankless because it is a very small hot water usage house. Could you please post some models that would work for my needs and some photos of the flue duct I will need and how it would connect from the heater through the ceiling? I want to use the closet the old heater is in now and it is in the center of the house so I cant exhaust it through an exterior wall to the outside.

 

Buy a cheap standard electric tank, and seal up the flue holes. There are no gas-fired tankless solutions that would make sense for a low volume user, electric tankless solutions aren't worth it (as outlined on your other thread) and the standby losses of a gas-fired tank are very high compared to an electric tank.