You have got some good ideas so far. A couple more, if I may:
1. If you have really good water pressure at home, buy a fireman's hose nozzle. They sell them at Home Depot and Lowes for about $20. You can adjust the spray pattern to a narrow blast of water to remove mud and crud.
2. A pressure washer is exactly what you need. As mentioned, you can find a basic electric washer from brands like Karcher and Campbell Hausfield for around $100. Home improvement stores, WalMart, tool stores, Harbor Freight Tools, Northern Tool and Equipt, Pep Boys, etc, all sell them. You can find plenty online. Heck, even the Pamida might have one.
3. More on pressure washers, ask around and see if someone is selling a used one. Lots of time, people want to upgrade but need to unload the old one. You might find a nice one for around $100-200. If you do buy used... and I feel silly saying this to an Iowa farm girl.... but make sure it works and comes with all the attachments so you can use it right away. A test drive will also help you decide if you can handle the power.
4. If you want to wax trucks, you really need a buffer or polisher. We talk about all kinds here, but you don't need to get all fancy. A used buffer, some pads, and some polishes and waxes will not cost a lot if you buy used. If you want to go new, a full kit will run you around $250-300. Used will be much cheaper but you will have to piece it together. Don't worry, we can help you with that if you decide to go down that road.
Hope all this helps. Holler at us if you need anything.