What you probably want to do is secure some kind of rugged armored vehicle in drivable condition which you can then alter and improve upon. My favorite is an armored personnel carrier, but a SWAT van or a security van or somesuch will probably do. Just look for reinforced windshields, some kind of armor, and heavy duty frames.
Then you take it someplace safe where you can work in peace. Almost anywhere can do if you’ve really cleared it out thoroughly, but my favorite is a fire station. You can get your vehicle inside where the fire engines would go, there are no windows or wooden doors for zombies to bang on, and it already has a bed. Just make sure you really CAN handle the area, don’t get yourself trapped inside.
Okay so now you’ve got something to work with, and a place to work that’s hopefully stocked up with tools you’ve been hoarding. You have been hoarding tools, right? Have a cargo unit on wheels to drag around and loot with. Grab every single acetalyne torch you can find. Get a welder, welding goggles and a shitload of batteries. Duct tape, hacksaw, jack. Keep your eyes open for hardware stores and garages.
Now that you’re ready to go to work, the first thing to do is repair your salvaged vehicle, or at least any parts of it you don’t plan to rip out. After that you can begin to make some simple changes that will increase the durability of your would-be deathmobile.
Move your fuel tank to the center of the vehicle, where it’s less likely to be damaged. If your vehicle has wheels on the frontmost row of squares, move them back one. Maybe add a bumper of some sort, possibly with spiked plating as armor for the extra damage. Get armor of whatever sort on every square, because the default vehicles only armor the outer layer of tiles. Exactly what kind you use will depend upon your skills, but try to get at least steel plating on every tile.
Get curtains on all your windows and doors so you can close them and break line of sight with the world when you’re trying to sleep. Add a bed if you want, but a seat is actually pretty comfortable for sleeping. Try to get cargo containers on every free interior tile for max hoarding.
Get you an electrical system. Look for electric cars to salvage parts from since they have storage batteries and solar panels. Get a good number of storage batteries in there, get some solar panels on the roof, and make sure your motor has a truck alternator attached. Get some lights on your vehicle. Floodlights and headlights.
Slap in some water tanks and at least a faucet. Then you just add stuff either as you find it, or as your skill grows enough to craft it. Things you want to get are a kitchen unit, a FOODCO kitchen buddy, a vehicle welding rig, and probably a vehicle mounted forge and chemistry lab eventually. You definitely want security cameras on the edges of the vehicle and a camera display on the same square as your seat, as soon as it’s available. Eventually maybe you’ll get a minireactor.
Weaponry will depend upon your skills, the mods you’re running, and what you’ve found. I very thoroughly recommend the Vehicle Additions Pack and the Tank Mod since they add a lot of different weapon options. Even without any guns though, you should be able to zoom past zombies and not have to worry over plowing through a few of them.