Deathmobile designs vary a lot. Here’s what I would do:
- Find a working vehicle, or a vehicle that you can easily get working. That means one with working controls and a fuel tank and an engine in yellow or green condition. You’ll also want a scissors or bottle jack to change the tires and a wrench. Swap out tires from other, nearby vehicles if necessary, and if the battery is bad or has no charge, replace that too. The exact vehicle doesn’t matter - a minicar, beetle, police sedan, electric SUV, whatever - just that you can get it running easily.
- Now that you have a vehicle, consider small improvements to it. I usually make a makeshift welder and a battery compartment mod, then get a hacksaw and start cutting out bits and replacing them. For instance, upgrading trunks to cargo carriers increases the amount of loot you can put in your vehicle.
- Keep an eye out for better vehicles. At some point, you’ll want to switch to something better.
Answers to your specific questions:
Any reasonable vehicle will have plenty of storage space and all all the water storage you’ll ever need. No need to prioritize those. Focus on armor and sustainable motive power.
Refurbish an existing vehicle. You want heavy frames, armored wheels, and military composite armor. Security vans and SWAT vans are good starting points, but military trucks and APCs are better. If you have the Tanks mod installed, Mobile Gun Carriages are also good. Humvees and armored cars aren’t bad. Actual tanks are okay but they are fuel hogs.
Your deathmobile is going to want a v6 or better engine, armored wheels, military composite armor, storage batteries, and solar panels. Keep an eye out for those. You will probably also want mini-fridges, mini-freezers, welding rigs, and water purifiers, so look for those or parts for them. Mechanics textbooks are also very helpful.
You can run a solar deathmobile if you keep the size down or don’t mind letting it recharge for a few days. I usually go with diesel until I hit Mechanics 8, then hybrid electric-diesel.
The size of your deathmobile is a personal thing. I like no more than 5 tiles wide and 8-10 tiles long, but some people build them much bigger or smaller. You can fit 4 cargo carriers, a box, 5 floor trunks, 6 seats, a minifridge, a welding rig, and as many stowboards as you feel necessary on a 5x9 footprint and that’s a huge amount of cargo space for loot runs.
Vehicle additions pack and Tanks give you more options for makeshift and improvised vehicle weapons, plus more sources of military composite armor, .50 BMG ammo, and extremely heavy guns.
You don’t need a storage area and fungus can’t take over your vehicle. Plus, most fungal sources don’t handle bursts of 0.50 BMG fire very well, so some anti-fungal pills and a turreted HMG is generally more than sufficient to tamp down on the worst of the fungal infections.