Doesn't really matter about keeping the IWH and boiler from the same company. Typically, at least from what I've seen, the temperature sensor for the indirect is part of the boiler's installation kit and all IWH have a well for the sensor. I've got a SuperStor Ultra that serves my needs.
I've got a Buderus unit that I like. I bought it on recommendation from the local installer who had one in his own home plus, the USA distributor is about 15-miles from my house, so expertise and parts are close, if needed.