Always deal direct with the heating installer....builders are notorious for subbing the work out to the cheapest contractor and then not paying them. You can also end up with a carp system.
You need to ensure you have sufficient water pressure and flowrate if you are going down the unvented cylinder route. Also consider fitting flow limiters to all outlets.
The SystemFit cylinder option is a good deal if you compare buying the standard unvented and adding the extra parts, provided the package includes quality components such as the RWC bypass, Honeywell zone valves, Grundfos pump, Danfoss controls etc.
Your builder is correct....the cylinder is designed to operate with a heat only boiler and just requires the addition of a safety valve.
However, if your system has been calculated to require a 28/30 Kw boiler then it's possible the supplied expansion vessel would be of insufficient size so you would require an additional vessel. Vaillant boilers (and many other manufacturers models) have considerable resistance through their heat exchangers. Again it's quite possible that the supplied pump will be insufficient. The unit appears to come with a Danfoss TP9000 programable thermostat/DHW clock...are you happy with that?
A system boiler contains an expansion vessel, safety valve and pump. They would be used with a standard unvented cylinder but you would need to add the zone valves for the heating system. Some manufacturers eg. Worcester allow you to add their diverter kit to the boiler to save external valves. Also note than many system boilers have insufficiently sized vessels and pumps so again additional vessels and pumps may need to be added.
IMHO the larger output system boilers are pointless for that reason. I also prefer to have the vessel, safety valve, pump, gauge separate from the boiler....you are not tied to the manufacturers own parts and availability when they fail and they can be sized correctly.
My preference for a boiler either heat only or system would be the Worcester CDi Classic models...based on previous reliabilty, lack of any issues, easy parts availability, excellent manufacturer backup.
I would also question the size of boiler mentioned. A Victorian terrace house unless 3 storeys would surely suffice with a much smaller boiler. It is also possible the hot water could be setup as a priority system allowing say 20 Kw for quick reheating of the cylinder. Then 20Kw or a little more may be sufficent for the heating too leading to a more efficient system.
(the builder that is).
