Taking the Ariston example (most probably similar)
- 10L Unvented - 2.8m in 15mm from the cold tap takeoff
- 15L Unvented - 4.5m in 15mm from the cold tap takeoff

So basically you need a PRV to drop pressure down, then a pressure
tank, then a pressure release valve & tundish with route to a drain
point.

Unvented can be a PITA in terms of added cost, components & space
required.
Conversely Vented can be a PITA re tap cost & "that drip problem".

The inline 5.0kW heater is smaller than any and should be enough for a
sink, it does require a 20A feed. If there is any chance you are going
to stuff a Saniflo you need to use a 32A feed. For that reason I would
use 32A cable, just in case someone comes out with a 6.0kW instant
heater which would be that very appreciated bit better than 5.0kW. I
think the 3.0kW unit is a dead duck for anything other than a Part M
toilet "cat weeing on your hands with something approaching warm"
after strangling the water supply with a flow restrictor :-)

Actually the cat might be a better bet, so plan on a velcro wall mount
and bottle to feed the cat (I am a cat lover :-)