AFAIK the ID.3 as default only comes with a Type-2 to Type-2 cable, everything else is an option.
Electricity prices have gone up a lot this year, when does your current tariff end with your supplier? See what the price increase will be once it does, don't forget to look at the standing charge as well.
Worth doing, as you are right it could be more expensive if you don't charge the car a lot. Although you should look to see if you can shift any power use to the off-peak pricing as well, e.g. running washing machines, dishwashers etc. as that all adds up over the year.
A commando plug is just a 3-pin socket really, but capable of high current as per the 32A recommendation, you can buy 32A commando to Type-2 cables, but they are quite expensive still, and as you put it charge dumb, and just use the inbuilt timer in the car.
Seems like a very cheap price, especially if he is using proper armoured cable for outdoor use etc. IMO if you have a caravan it is worth having anyhow, it's not like it will only last a year, and the same goes for the EV, any money spent on a charging point or device will last you beyond this vehicle, and probably the next couple after than, unless you move house, or we get cheap V2H or V2G sooner than anticipated.