You're on the right track, however there is more to this question than whether something's available yet. The first issue is that since the Stirling would benefit more from the higher temp differential, it needs to receive the heat first with the boiler receiving it's waste heat. A really good Stirling 'could' start making usable power in the 650 F range, so that would be the temp of its waste heat which is still high enough to operate the boiler. Now the only problem becomes sizing the quantities produced vs. being used in each stage to maintain those temps. This would allow any heat source capable of sufficient quantities above that temp to efficiently supply the heat, electricity and possibly the cooling for a house. In addition to that, a good heat pump circuit can increase the differential by lowering the cold side another 60 deg from ambient or from a well. With a COP of >3.5 and these temps, the Stirling would make a significant positive power output. This is in development right now but it isn't available off the shelf yet because of the next reason.
The real delay is purely financial. The current perception of Stirlings is that they have been tried so often that that many failures couldn't possibly be incorrect. This isn't exactly accurate. New designs, materials and computer modeling have brought the virtual development capabilities to the masses. In my personal experience, it doesn't matter what the computer modeling results show, that perception problem still exists for the stage of raising prototype money. This has proven very expensive, since professional engineers, patent lawyers and machinsts didn't come cheap. So anyone looking for an off the shelf Stirling, you might consider supporting that stage of development.