Radiant boiler installation

I am converting from gas steam to radiant under floor (heat only) system in an old victorian 1920's home. I removed all the steam piping and rads. I am doing thermofin c plates with foil and insulation in the bays and I figured a total heat loss of 110k btu for structure for 2 floors. I have 3 floors but can only get under floors for first and second floor.

I dont need much heat on third floor (bedrooms) and was thinking of doing radiant with wall hanging european style rads by droping pex down steam pipe holes. If not, then electric space heaters. Radiant baseboard could be option as well but seems very cumbersome.

I am at the point where i am hiring a plumber to install a new viessman vitodens 110w boiler, piping, and uponor manifolds. He has done a few radiant systems over the years but not a viessman. The plumber gave specs to local supply house (supply house guy is designing system) and came back with supply list for system. At first they wanted to do combi unit with dhw option for 3rd floor baseboard. This would involve mixing valve and more mechanical.

My thought was heat only with separate zones for first and second floor with space avail for zone for third floor and basement in future if needed. This would keep the piping very simple with just primary/secondary loop, closely spaced tees, low temp only and simple schematic.

Questions:

1. Which system would work best - low temp radiant only or adding mixing valve for high temp baseboard?
2. Is plumber experience ok if never installed specific brand boiler?
3. 3rd floor option - which to use?
4. Low loss header - part of supply list.
5. Primary/ secondary ok?
6. Boiler choice ok?

Thanks
Mac