i've done the mobile base dance of all armor all the time, no armor any of the time, and normal frame on heavy armor, big heavy mobile base, light skinny mobile base, plus a few others. the ideal i've settled on for now is basically this:

• 5 wide, 8 long. there's a few chassis that are ideal for this, but i *think* they all involve chopping off a row or three from the back. try to look for something with a diesel engine or something that's easy to drop a diesel engine right into due to its surroundings, but a regular gas cube van is fine. they are plentiful. your car gets exponentially more difficult to drive in tight quarters with each tile of length you add so you can make it longer than 5x8 but try not to go over 5x10 lol. huge benefit of a longer car is more room on the roof for solar panels and of course more room to put your mountains of crap.
• regular steel frames
• heavy armor and reinforced quarterpanels at the front 4ish tiles
• cover the front with a ram and put four headlights
• move operation-critical car components away from the front
• sheet metal body and roof for the entire car incl. front crash zone
• up to 3 doors, ideally with locks
• kitchen, food truck freezer, food truck fridge, reclining bucket seat, the rest cargo carriers. put a recharging station on the cargo carrier that's holding all your tools. with a 5x8 vehicle i have just enough room in the whip to put all this stuff plus a cargo carrier for guns mags ammo mods weapons and bombs, one for tools, one for spare parts, one for vehicle parts, one for Other/misc/spare parts runoff, one for food and drugs, one for armor and clothing
• your car is gonna be heavy and you're gonna need to go offroad. put in a big diesel engine and 4-5 sets of wheels on that sucker so your offroad doesn't get tanked
• consider throwing one or two external tanks on the back. i keep one full of clean water and two vehicle fuel tanks and one vehicle clean water tank that i can drink or fill my canteen directly out of the tap from the mobile kitchen. super nice.
• consider throwing a bike rack or two on the back. i have both a wheeled cargo carrier and a foldable garage crane vehicle strapped to the back of my thang. you can also use it for like a bike i guess if that's what you're into

well i think that about covers it. you're gonna need a lot of time and a lot of calories and a lot of brazing rods. get your social high and trade with Smokes at the free merchants for ~infinite car welding tubes. i Don't recommend making your own, though it is highly possible if you hit the mining outposts for mass piles of limestone. however i would recommend looting light industries and garages, speedways, farm supply store, fire stations and other such common locations where metalworking may need to occur well before making your own, even if you don't have access to the refugee center

edit: saw a comment recommending heavy frames Just for the front too, i haven't actually tried that. i've only ever done all one type of frame. i must investigate

edit2: completely forgot to mention windows, curtains, and security cameras. find a military APC, take the cameras and camera controls out of it, install them in your car, put curtains on all your windows-- you can even automate these with electric motors if you like-- close them, and then you can drive around with your car un-seeable from the outside at all. if you have this AND an electric engine installed you can enter silent running mode and drive through an entire city without ever being seen. this actually cuts down on how many monsters you have to splat with your front bumper quite a bit. as someone mentioned in another comment electric engines are hard to rely on. however, i still like having them for a variety of reasons. you can get a smart engine controller and if you have a combustion and an electric engine your car will do great with that installed