Ok so I'm about read to take two solar powered vehicles and turn them into one better one, but have never made a vehicle before. The most I've done is replace tires.
I'm planning on making it 5 spaces wide, idk how many across, with 3 trunks, armor on the front, 2 electrical engines, as many solar cells as I can fit, 2 storage batteries (with the other 1-2 for other uses such as electricity for a base) 4 wheels, and 2 doors. If I wing it and keep it simple should I be ok?
You'll probably have to switch electrical engines to LARGE electrical engines. Having armor, even wooden, will significantly increase your weight. My tiny minibase weighs 452 kg and 2 electric motors just brings it above 50mph safe speed.
1) What are some limits for putting things on the same tiles?
That's a simple question with a broad answer, but for the most part, no two items of the same type (armor takes up the armor slot, etc) and No two items of the same name (for items that don't have slots such as water faucets). A notable exception is the "vehicle" batteries. A storage battery, water tank, plasma tank, gas tank, and small gas tank take up the "fuel" slot; but you can happily install a truck battery, motorcycle batter, and car battery on the same tile along with the previously mentioned storage tanks.
2) Is there a ratio of width to length I should keep to?
Not really, weight is the more important factor here. Just remember it gets harder to turn the wider your vehicle is.
3) How does armor work?
From what I've seen, any damage that vehicle tile takes has a decent chance of being done to the armor instead of the other components of that tiles. Armor does not work on vehicle equipment mounted externally, such as solar panels. (This doesn't seem to be the case in the newer builds). Remember that damage can travel through your vehicle, not just the tiles being hit.
4) How does enclosing your car work? I heard that a board OR roof OR door means that it's protected by the elements, but don't you need a board and a roof?
Nearly all items that obstruct movement (like boards) count as an "IN" tile. To make a tile enclosed, that specific tile and the four tiles around it need to have a roof or be otherwise obstructed.
5) How do doors work, can they be alone on a tile, or should they have a floor/roof and can I armor plate them?
Like pretty much everything else, doors need to be constructed on a frame. External doors act like an "IN" tile when closed, but you can put a roof on them. Internal doors act as a roof even when open, but block sight when closed. You can plate them with armor. You cannot build a floor on a door tile so that's moot.
6) Any tips?
Before going big, I find it best to create a small, efficient vehicle that you can drive around as a cargo carrier. There are lots of car wrecks on roads, you can install a welding rig on your car to dismantle wrecks for building material. Remember that stored items also count towards your car's weight.