I've done all this stuff, here's what I have to say...
Playa vehicles do tend to coast along at better than 5MPH and no, it isn't a carb issue, unless you want to try to get your idle speed down to something like 2 or 3 hundred RPM, where your alternator won't charge and your water pump wont flow enough water and your oil pump won't make any oil pressure. At a nice normal 600 - 750RPM idle, you do end up rolling a bit fast and have to drag the brakes a bit. You could go to the trouble and expense of finding super-low gears or just buy a set of brake pads every few years for $20.
"Odd suspension settings for snow" isn't even an issue for a playa mutant vehicle and it's extremely unlikely you'll even stumble across such a thing.
Manual steering has absolutely zero to do with whether a vehicle can be flat-towed (on it's own wheels). At super low speeds and a full deck of people, power steering is nice. Personally I find automatic transmissions a lot nicer for constant starting and stopping and low-speed crawling around, like you do in an MV on the playa. If you have the skills to build a mutant vehicle, you can probably handle unscrewing four 1/2 nuts and dropping the driveline if you have to flat tow it.
Automatic vehicles make it a hell of a lot easier to relocate the driver's position.
Remember that a vehicle has to be currently licensed and street legal to flat tow it just as if you drove it. If it's not gonna be a road vehicle, you want a trailer.
Street legal cars CAN be mutant vehicles, but be advised, the DMV keeps raising the bar and it's pretty damn hard to build an MV that will get a playa license that is still a real, street legal car.
The best chassis for your project depends on two things: What you're building and what you happen to find a deal on. An old Chevy van is indeed a great platform, unless you have a free mini truck that runs. Both suck if whatever the hell you're making just doesn't happen to fit a big square chassis.
Front-wheel-drive vehicles can work well because the whole engine/brakes/steering package is one modular unit that can even be cut off the rest of the car. The rest of the vehicle design is then wide-open to your imagination and/or desired configuration.
Electrical power is a big thing to thing about. The stock single alternator, running only at idle speed, will be inadequate for powering the amount of lighting the Burning Man DMV expects you to have if you want a night license. You can just run a Honda generator on your MV but afetr doing that for years I find it less trouble, cheaper, and even quieter (since the MV engine's gonna be running anyway) to simply run two or more alternators on the vehicle's engine, coupled with a few batteries and inverters.
Fuel-injected base vehicles have some great advantages over carbureted ones for playa MV use. First, the playa is almost a mile high and the electronics will automatically adjust for it, and the rig will run perfectly.
Second, if you add at least one extra alternator (I highly recommend that you do) the fuel-injected car will automatically adjust it's idle and not just die unless you give it gas every time you turn on all your stuff.
If you're using a big V8 engine vehicle you won't likely have trouble with alternator load at idle even with a carb, but I'd avoid that because a carbed V8 idles through a LOT of gas. A little injected 4 uses hardly any and still makes more power than you'll need.
Too much is not enough, and when in doubt double it.