There are block heaters available operating on either electricity or fuel.
The electric ones are usually low-power (300-700W), the lowest-power of which (300W) just heat the block externally and the best of which (700W) heat the coolant and have a coolant circulator pump. These are ideally combined with an interior electric heater (1000-2000W). I have a 700W coolant heater and 0W/1100W/1900W interior heater in my 2016 Toyota RAV4 hybrid. I also have these connected to a battery charger, so the same cord provides battery charging, coolant heating and interior heating.
The fuel-operated block heaters (at least Eberspächer and Webasto are available in Finland, the country I live) are much more powerful, 4-5 kW, and can heat both the car interior and the engine. Unfortunately, the fuel-operated heaters require about one minute of driving for each minute of heating because they use up the battery charge. So, if you heat for 30 minutes, your trip should last 30 minutes or else your battery charge depletes in the long run. Of course, this can be solved by installing a battery charger, but then you need the cord and it's a good question whether it makes sense to have a fuel-operated block heater with battery charger cord or just go for the electric ones.
However, you may find that if your cars are old, nobody may have in stock the exact block heater parts needed for your cars. Usually, these devices are installed on new cars, not on old cars.
My advice for the next car purchase would be to select a gasoline model. Not only are diesels dirty, they also may likely be forbidden in the future in many densely populated places due to exhaust emissions, and diesel engines are less reliable and more expensive to fix than gasoline engines. Add these to the cold start problems of diesel engines and you have a loser. So, choose gasoline! And for good cold starting, choose a block heater for the gasoline car.
I have never experienced any cold start issues at -10C to -16C on any gasoline powered car. Heck, all of my cars even started up just fine at -20C even though I occasionally didn't bother to schedule the block heater use.