Man. This thread has been a real crash course in home heating systems, and I truly appreciate everyone's contributions. I've even been credited with thinking like a wet-head, which I think is a good thing.
Ironically we jettisoned panel radiators from our plans in favor of the more budget-friendly fin-tubes. You see how that worked out for me. For the same cost I could have had an appropriate sized boiler and panels, perhaps even saving the cost of a buffer tank. 20-20 hind sight and all that.
So I think I'll negotiate a Boiler-Buddy and maybe a smaller boiler with the original installer, and then watch the performance of the fin-tubes for a while. Our upstairs comprises two bedrooms which are kept cool anyway, and a hall/sitting area and bath which pretty much heat themselves from heat rising in the stairwell, so it all may be okay.
But to anyone new to the science of modcons and hydronics, my advice is to do your homework before calling a heating guy, not after as I did.