The simple answer would be: sure, you can use a conventional H2O heater -- in theory.
Code requirements may say different, if they apply in this situation. I would recommend the later hybrid though. The longer, more detailed answer requires many questions first and will also require an absolute pro in retrofits.
So here you go, and I am sure I am forgetting something:
Are you going to install the PEX tubing in an insulated/radiant subfloor such as Roth radiant panels? If so, insulation is very important for radiant heat to go in the specified direction. You live in a somewhat mild climate, and I take it you are replacing the flooring, so if all you have for energy is electricity then the thin electric radiant mats would suffice and not make your meter spin wildly out of control. The design requirements for radiant floors (which are a luxury) are deep, so I recommend you seek out a pro, not a plumber who says they do radiant floors. You must use someone who has the retrofit knowledge to design the correct system taking into consideration all the parameters your building demands. Are you re-insulating the building? New high-efficient windows? I recommend doing heat-loss calculations to determine proper sizing of the heating system. Do you need refrigeration for the summer? Does anyone in the house suffer from allergies or asthma? If you are considering a staple-up system for the PEX tubing, I would not! It is a waste of time and money.
You may want to consider installing radiators, which nowadays are quite appealing and can handle higher supply temperatures and are much easier to retrofit and more cost-efficient overall.
There is a whole lot more to consider, but not knowing the climate and what is going on with the rest of the building, that's all I have for now. Please feel free to reply with additional parameters so I can have a better understanding of what's going on.