I have a Buderus unit in my townhouse that has been working fine. The US headquarters for Buderus is in Londonderry, NH, so both expertise and parts are close by. The biggest thing is to choose the proper size. Even though the units can modulate, if it is too big, it is too big and won't be anywhere near as efficient as it should be. Do NOT just replace with a boiler the same size, in most all cases, it will be 2-4x oversized. To get the best economy out of the thing, it needs to be right-sized, and that means it should run all the time during the heating season at just the right heat output that matches what's lost to the outside. If it is too big, it turns on/off, and each cycle wastes energy, plus, each cycle wears things out faster. I have bascially, their smallest unit (and a 60-gallon indirect so I can fill a big 6' soaking tub), and it could stand to be smaller. It did cut my heating bill by about 1/3 over the old unit (the rates keep changing, so a direct comparison is harder just using costs).
If a boiler is properly sized, on the coldest design day, it would run 100% of the time to keep the house warm. A boiler that only ran 1/2 time would be probably at least 2x oversized, and no, it isn't more efficent at that. Being able to modulate means it can run close to 100% more of the time as the load changes throughout the heating season. Obviously, it can't run 100% every day, but the goal is to let it do that as much as possible, and getting the right-sized unit is critical to that and saving money. Plus, the smaller one starts out less expensive, too in parts.
If they haven't done a room by room heat load analysis, they're doing you a disserivce.
I have no experience with the HTP unit, it might be great, just don't know.