The backpack really is a slightly modified crop sprayer, but it works. It holds about 16 litres and fits in a car nicely; right behind the passenger seat in the footwell.
You can buy trolley systems where you put a 25 litre barrel of water on it, stick a tube into the barrel and off you go.
There's pros and cons with each. The trolley systems can be quite heavy even without the water, and may not be suitable for a car. For example, you'll have to put it in your boot; and then to get it out - if you still carry ladders - you may have to take the ladders off the car first.
With working from a car, I prefer backpack to trolley (I've used both). With working from a van, I prefer 100 meters of microbore. I don't really rate trolleys 'cos I never got on with mine (Cleantech trolley 25 to 50 litre one).
Personally, if you can, dig deep - borrow - and get a van system; but we worked quite effectively for a number of years using a back pack; me doing tops only - or anything awkward, leaded or georgian; and Wor Lass tradding the ground floor/insides.