jit

i am electrical engineer , wanting to understand heat pump schematic and how system perform. i am in the process off having my heat pump installed. i had 3 heat pump engineers turning up with all different design. the house is fully insulated . 165m2 heating space and 7kw heat lost. 300l new cylinder (not 3m coil). ground floor UFH, 1st floor radiator circuit, 2nd floor radiator circuit (loft) from my reseach and what i understand. i want to install 7kw vaillant heat pump, open loop and no buffer/volumiser. engineer1 said. install bufffer. and new tank engineer 2. install bufffer. and new tank + zone radiator circuits engineer 3. suggest open loop system with no buffer and use existing cylinder with plate heat exchanger . from my research and what i understand . i want to avoid buffer and have open loop installation with balancing valve to control flow. i am not sure if i should plate heat exchange on existing cylinder as it 10 people and 3 bath. and do i need volumizer as their is plenty of volume being 3 circuit. i have boiler room outside and all the pipework run separate . the pump can be installed on same wall. what would be the good and high efficiency set up?