No reason why it shouldn’t have a cavity, albeit a narrow one given the width although at 250mm wide it would be on the narrow side.
Those plugs at the base of the wall look suspiciously like an injected damp proof course.
Windows look alright, rub them back and redecorate. If they are hardwood have a look at linseed paint, it is significantly better than most others particularly for wood since it penetrates the material. It is more expensive but will last a lot longer.
The bead between the pane and the frame; to be honest you could probably leave until you have the money to replace them, you might be able to find a firm who can recondition them if you’re set on doing something.
The cylinder size will depend on how many people are in your house, a 200L one won’t go too far with four people but if it’s just the two of you it would be ok.
Is an injected dpc a problem? I didnt see any obvious damp in the house but the walls needed replastering in some places.
Only me living there, one bathroom only, currently no shower fitted. So I guess 200 litres would be enough. Id probably just go for whichever option is cheapest to do. A cylinder would fit straight in place of current one and keep existing boiler whereas a combi boiler would need additional pipework running to it. 200 litre easyflo cylinder about £700 online ready plumbed. Fitting about the same again? Compared to combi swap about £2500? The bathroom is in desperate need of refit but Id have to sort the hot water solution first.