Advice needed, cleaning a few years of dust off a car.

Take whatever advice you want.
One thing is.. you might want to use compressed air to blow out the 'seams' to avoid mud in the crevises .

Things to consider anything you rub it with residual dirt might scratch. If you hit it with too hard of water you could damage something. Of course all this ****'s obvious right so let's move on to the next. However if you do use some pressurized water and I mean you know like the thumb over the hose or a little more but not full on pressure washer turned up to high, consider that aside from the initial blast Point all of that sideways deflecting water will actually blow the dirt off in a sideways fashion and is better than rubbing anything at all on it with the dirt still there.... so
Everyone saying not to use anything to rub on it and use a stream of water or bucket of water and soap ..or whatever, are correct.
What I like about the cheap Harbor Freight pressure washer is that it comes with the soap dispenser to put on the end made for washing cars so it will not push too much pressure and comes off like a thumb over the hose kind of a deal and then you can adjust it at the head once you remove that to rinse so that you're only putting a fan of water across it.
I took more time than needed for this.
It's dirt on a car, use your head... it's easy.