Does a block heater make sense for a gasoline-powered vehicle?

I have a small fan heater (500W) that I put under the bonnet of my car and run it for a couple of hours early morning.

The car starts first time no hassle, but if I leave it to get cold and then try to start it, it takes 4 or 5 attempts and hammers the battery and starter. It is a 2.2 diesel...

So, I went the cheap route and it makes such a difference, try it and see.

As I currently reside in an area which experiences cold* winters, and do not have access to a garage, I started considering getting an electric block heater. The design I have in mind is installed in the coolant circuit. It consists of a heating element, a coolant pump and a mains power supply. The car is plugged in up to 4 hours prior to being used, and the device will heat the engine coolant to operating temperature.

Will the block heater provide a real benefit here (and possibly in other areas) on this kind of vehicle?

Absolutely, especially for a heater in the coolant circuit!

There are several types of block heaters:

• A true old-fashioned block heater installed in a freeze plug. They are typically reasonably good.
• An "oil heater" attached to the side of an engine block. These typically have rather low power (example: 300 watts) and the main benefit is that the oil viscosity isn't as big issue when starting the engine since they heat the oil. You'll have to wait for a long time to get heat from the engine, though. I used to have one in my previous car (2011 Toyota Yaris), and it does indeed make it a bit easier to start.
• A heater installed to the coolant circuit. Especially the ones that have not only a heater but a circulating pump, the power can be quite high (example: 700 watts). I have a 700 watt unit in my current 2016 Toyota RAV4 hybrid and it's very useful.

When installing a block heater:

• Install electrical wiring to the footwell of the passenger so that you can later install a cabin heater. This cabin heater will mean the windows won't have any frost when starting the car, and the inside of the car will be warm. These typically draw quite a lot of power though (example: my current cabin heater is 0W/1100W/1900W selected by a switch and I use it at 1900W).
• Consider whether you want to permanently install a lead-acid battery charger so that the lead-acid battery won't drain if you drive short trips. I have a 12 volt 4 ampere lead-acid battery charger in the same wire as my block heater in my 2016 Toyota RAV4 hybrid, so my lead-acid battery will never get drained. As this car is a hybrid, the load on the lead-acid battery is not that big, but it's useful nevertheless if using the heated windshield often as it's powered by the 12-volt circuit and not by the high voltage hybrid circuit.

I use the heater with a 2-hour timer. The 700 watts will heat the engine well enough in 2 hours, and the 1900 watts will heat the inside of the car well enough too in 2 hours.

4 hours is excessive in my opinion unless the heater is underpowered ...in which case it won't be of much use in the coldest days.