With 24 kW available you would be foolish not to go to a quick recovery
cylinder.

I'm probably going for something like a DPS heatbank with pressurised
primary coil, provided I can persuade them to make a 150L or 180L version
with the double plate heat exchanger (they only list the 300L with this as
standard). I need the double heat exchanger as I think the projected 30lpm
to 42C of the single is too low. I need pressuried DHW and heating circuits,
as I will be converting the loft and putting in a shower room and radiators.
Whilst in general, I'd prefer an unvented cylinder, the location in the loft
with no obvious safe and visible location for the pressurised vent makes the
heat bank more sense, despite the fact that it will reduce the opportunities
for the boiler to condense.

If you are fitting just two-port valves, then it is a simple matter
(maybe using a relay) to convert the system to quick recovery
priority.

I'm more likely to wire it up conventionally and put in relays if I find a
problem in practice, which I don't anticipate.

Christian.