Your radiant system doesn't need antifreeze in the water, and your car radiator doesn't needed it but it's sure handy to have if the temperature of either dip below 32 degrees.

The water pressure doesn't come from any water heater, it comes from the city. What ever pressure the city maintains in the line will your water pressure.

A tankless water can be purchased to handle radiant and domestic hot water and uses a priority configuration. This would switch all the hot water made in the heater to supply hot water before letting the hot water go to the radiant floor.

many discussions take place on houzz all the time. If you google "tankless water heater houzz" you'll find dozens.

There are pitfalls using a tankless water heaters. Not to supply both domestic hot water and radiant heating but other minor pitfalls using a tankless system.

“I have 2 questions. 1st - Does the radiant heating system need antifreeze in the water?”

Yes, if you want to protect your imbedded radiant loops from freezing and ruin - if you live in a cold climate.

“Will 1 tankless unit produce enough hot water and pressure to handle both the radiant heating and the household hot water needs?”

Yes, but it has to be properly designed by a professional that knows what they are doing. It’s a very specialized professional that can do this and get it right to not only work but to work properly, reliably and efficiently.

IMPO

SR

fsq4cw has previously demonstrated his knowledge as someone who works in the HVAC industry. Consider his suggestions as being informed ones.