Waste of money,you don't need the pump.
Just think of the UFH as installing a new rad,coupled to either a TRV or coupled to a separate stat.
You're over complicating a very simple addition to your existing system.
In terms of your existing boilers capacity,and whether it will cope,check the KW/BTU output of your boiler.
Find a calculator online that will work out the extra capacity of the UFH,and do a comparison against the extra load of the UFH on the Combi.