A few things to think about here.
First, how cold is the cold water that comes into your house in the winter? In the north where I live water comes in at around 40 degrees F in the winter so I'd need a much bigger tankless system to get it hot enough than if I lived in the south and water was 50 or even 60 degrees coming in.
Second, how much do you need to heat your house? I don't know the gpm that would be sent to your radiators, but I'm assuming it is a lot. While you can pretty reasonably not run multiple showers and the dishwasher at the same time I think it would be a lot of trouble to ensure that your heat doesn't go on while your shower is running so you'll need to size the unit to be able to feed both the radiators and the shower at the same time.
Both of these could lead to your needing a fairly large unit that might be expensive. One of the reasons a unit like this can be pricey is that gas pipes generally need to be re-run to install it, and you need to install a solid vent too. I'm curious what sort of vent Bakari put in for his 1.6 GPM unit, probably nothing too huge, but if you're heating the whole house and need a big unit you will need a big and well installed vent to deal with the exhaust. If you can deal with a small unit you might also consider an electric unit, electricity costs more than gas to heat, but it's super simple installation (for a small unit where you don't need to add a new breaker) could make it worth it.
Another thought that might be good for you is to look into solar hot water/boiler combo. These systems are the most cost effective way of putting solar on your house and if you live somewhere warm they have the potential of providing hot water and heating with no ongoing costs (but obviously a large upfront cost). There is so much variation on the cost of these that if you're interested I really recommend talking to someone locally who actually installs them so they can tell you what works best for your climate.
You could also just stick with your current heater (that's what I'm doing) for a while longer.
Whatever you do, definitely put in 1.25 GPM shower heads like MMM recommended, they will reduce your hot water flow needs and let you buy a smaller system (whatever kind you choose).