Welcome to the CRVL Forum
I'd say you're off to a good start. Do you have a privacy curtain set up just behind the front seats yet ?
For your cot you could get a foam rubber egg crate mattress cover (twin size) at a store like Dollar General and fold it in half to have a foam mat for your cot. I've done this and taken cheap beach towels and laid one end across the top of the foam rubber, then under the cot and back across the top. Your weight on this when in the sleeping bag will keep everything in place. (I've used 3 of them cutting holes in the middle towel for the legs to fit thru) My cot is aluminum and has straight vertical legs. The foam is comfortable and a good insulator if it gets real cold some night.
For my curtain I got an expanding shower curtain rod with rubber feet at both ends. Set it up to grip at the roof line, and had wire shower curtain pins on it. I hung a beach towel sideways across the Van. (using the wire pins to poke holes in the beach towel)
This way you'll have some privacy and a comfortable cot to sleep on.
Once you're ready to start building, make a paper template of your floor from the front seats back.
You can use this to cut plywood to fit over the floor and screw into the metal floor in enough places to hold it steady. Then you can carpet that or paint it or put whatever floor covering you like on it.
Building recycle places and cabinet shops often have older bathroom cabinets without the lavatory top
with it. You can get one of these cheap and make a plywood top and set up a simple kitchen with that. This time of year you should be able to find ice chest in yard sales for cheap. 36 to 48 quart size work nicely. I've found a couple of those plastic 5 gallon Jerry jugs to carry water in so I have a water supply.
Below are a couple of links you may want to look thru. They can make your life on the road safer, more comfortable, and convenient It's all free.