What should you start buying to live in A vehicle? As little as possible. Money in the bank is peace of mind on the road. It will happen you will need to hit the emergency fun someday. So First make a list of the things you Gotta do. #1 you’ve got to eat, #2 you’ve got to sleep, #3 you got to poop and P. Then make a list of the things you want to do. these can be put off until you have enough saved for an emergency fund and then you buy those things. And with a Prius your space is limited so it would be best do you be on the road for a while and see what you really do and what you can do without. #1For eating thrift stores are full of $.25 forks and spoons, $.50 cooking pans etc. the stove will be a major purchase but craigslist and Facebook marketplace often have such things for you. Check them out that there’s no leaks in the gas system. I wouldn’t recommend one of the old liquid gas pump stoves of yesteryear. But Coleman propane stoves are just about indestructible. You don’t need fancy bottles for water. Walmart sells purified water and spring water for less than a dollar a gallon. A simple milk crate is designed to hold for those jugs. Not knowing what you’re eating style is can’t give you advice on that food exactly but Bob has information on YouTube. #2 Then for sleeping thrift stores again blankets, exercise pads, cots and mattresses - you might have to look for a couple of months but they do come up on craigslist etc. also. #3 Then the pooping in peeing you didn’t mention your gender and that can make a difference. Believe it or not Bob has videos specifically on this subject. The famous 5 gallon bucket is the way to go. 5 gallon is obviously bigger than needed but we older people can’t get up and down as far as others. smaller buckets work too. For a while I had a 5 gallon bucket in the bottom I kept sTP and other things needed and then put a smaller bucket with the bag and for number two on top of those things. Also the pipe insulation foam they make fits right around the lip to make it more comfortable seat. There are several companies that make a seat and lid that fit a 5 gallon bucket. I went to the next step in my new rig I built myself a wooden party cabinet with a shelf that holds a smaller bucket 5 quarts and underneath storage for TP cleaning supplies etc. As a man I pee in a jug and poop in a bag in a bucket, it works. They also make a device for women that’s sort of a custom shaped funnel so you can P in a jug too. But I gave you the best advice in the beginning don’t spend more than you have to. To put it another way plan the cheapest way to meet these needs then as you have the money you can make them more “comfortable”
Sorry I didn’t mean to write a book. Best of luck to you hope to see you on the road.