Bigger vehicles can be handy for having always powered tools (mostly food dehydrator and water purifier for me), somewhere to drive from that counts as ‘inside’ incase of acid rain while miles from cover and just flat out more storage. Later game its damn handy to have 5-6 cargo carriers installed on your vehicle so you can lug about half a town worth of stuff. Generally though, a huge vehicle is more a vanity item than actually being useful, 1 or 2 tiles wide is best for being able to navigate past crap on the roads, 4 tiles wide (so normal car size basically) is still semi maneuverable while having decent storage capacity with some modding and being a safe place to be in acid rain. Currently im running a modified pickup (5 tiles wide) with an extra layer of frames and armor on the front as a ram, extra layer of storage at the back and powered tools replacing the back windows. Its got more storage than id ever need, all the powered tools id ever need, 4 fuel tanks and a buttload of storage batts, it is though, just a little to wide, so I often have to ram cars off bridges so I can get past. If you are smart and willing to do without some of the onboard powered tools, normal car size will do you fine.
I have played a little with mounted weapons, but cant make them work, the best ive got is having the laser rifle I mounted on the front of a car target me and beep. Couldnt make the damn things fire at anything (including me though). In the past they have happily targeted the player if he/she/it was in range, so I believe many/all actually dont do anything at all atm besides look cool.
Laser stuff has the advantage of being silent/very quiet as well as ammo being fairly common if you have a UPS, generally though, rifles are less hassle and do more damage. Another good reason to have a vehicle with lots of storage is to keep ALL the ammo you find in one trunk as well as a differnt weapon for each caliber in another, after raiding a few gunstores/basements ammo really isnt a worry.
Manhacks work just as you say, you activate them, they become friendly and attack your enemies till they die. If you can find a goo canister, you can (a)pply it to drop out a friendly blob as well. Both arent that great against Zs as they die too fast to be useful as fighters however they can be damn handy for dealing with fungals.
Due to the game being updated multiple times a day, the wiki is pretty much always going to be out of date. Small price to pay 