I think a boiler with an indirect WH would work out best. Some of the companies make indirect WH's with dual coils, one for the boiler, and a second for your solar input. A boiler with a variable fire rate could match your low heating requirements, but still ramp up when called for to provide DHW. You probably only need one with a small output. A condensing, modulating boiler costs more, and you might find the break-even point doesn't exist, but as energy is only likely to rise in cost, that's hard to predict. Personally, I had a Buderus unit put in that has proven to work well.
Most tankless are not really designed for space heating requirements. Not to say that you can't make it work.