Seeking advice on upgrading heating system

[xenawales]

Hi. Apologies for what may seem a rambling post - I'm currently trying to improve my knowledge and assess my options and would be grateful for any thoughts.

I live in a large old property which is currently heated by an ancient regular oil boiler (Ideal De Dietrich Falcon GT) in the basement which feeds a large bore single pipe system. There is no hot water provision. The burner has been upgraded to a modern Firebird RDB1 21.6 - 32.5kW, and is controlled via a Hive system which also has a HW channel (currently unused).


We are renovating the property and want to renew the central heating pipework and radiators, plus install underfloor heating. I also need to think about hot water provision, and am leaning towards having a hot water cylinder (rather than combi), so the system would look like this:

which would be wired up as described here:
https://www.flameport.com/electric/central_heating/heating_wiring_S_plan.cs4
using the existing Hive controller for the rads and HW, and presumably an additional Hive zone for the UFH.

It would be my intention to get the radiator system up and running first (so we have some heating this winter!), followed by the UFH and HW (probably next year).

Questions:

1. Can I continue to use my current boiler on the new system until we can afford to replace it with a more efficient condensing model? I can't see why not?
   
2. I have been trying to find a nice 3 branch manifold so I can have all 3 motorised valves tidy in the basement. However, I can't seem to find one, so how do people do the branch connection between the pump and the motorised valves tidily?
3. The expansion pipe back to the attic tank currently comes from the highest part of the existing rad system. Will it still need to come from the highest part of the new system? (I assume so...) Unfortunately that is now the other end of the property!
4. At the moment, the hot water taps are all of the instantaneous hot water kind which works well, and I am inclined to leave it that way as for the amount of hot water we use, most would be left in the pipes (same as cold water feed washing machines are deemed more efficient). The HWC would only really be used for baths. Is this folly?
5. Anything else I need to consider at the mo?

We do have a plumber available for advice, approval of the design before commencement and undertaking any necessary works, however we are on a strict budget and will be intending to do the bulk of the work ourselves (builder and ex-engineer). At this stage, I'm just trying to get it all straight in my head so I can make sure I'm going in the right direction and have foreseen any issues.

Many thanks in advance.