I'm in Kalifornia and deal with the Crown Holocaust all the time. I have a Class A with many endorsements and still work commercially driving passengers around. Don't over think this. As a privately owned and registered non-commercial vehicle you're exempt from ALL the crap they try to get you on. Just buy what you like and get the best deal on.
Take the seats out, cover/remove the School Bus signage front and back, and especially cover/remove the red crossing lights, and the district/company commercial names on the sides, any TCP/DOT numbers too, and you're golden. As long as you're registered and have/show private plates on the thing and carry liability insurance no one has any jurisdiction over you, Period. I wouldn't even sweat the RV/MH registration thing. If you can get it registered as a simple "automobile" "bus body" like mine is you won't have any trouble or have to show/prove the 5 items installed making it an "RV". Keep it Simple.
All the rules are aimed at Commercial Operators. That goes for license requirements, Emissions and engine regulations, and as long as it's under 45ft long I guess they can't touch you here either. You can drive it with a clear conscience with a regular old Class C normal car drivers license. Perfectly legal. School buses aren't over 40ft long anyway.
I agree completely that if you drive it in a safe and sane matter you're mostly invisible to LE and others. Most everyone will buy a bus and drive it home with seats in and sometimes with all signs and lights still in place. Very sketchy from a legal point of view but being invisible usually helps a lot.
Anyone asking me gets the lecture to cover/de-school bus the lights/signage etc. and carry with them a one way short term trip permit/(registration) which is good from issuing State for X number of days to get it home. As well as having in hand Proof of Insurance as well as a copy of the Bill Of Sale to prove ownership should be all you need to get it home, even with seats still in place. It's a Ferry Trip and most LE will cut you slack as long as they see you just passing through their jurisdiction on the way somewhere else.
You're in Crown country and it might behoove you to look around for a clean and well maintained Crown to convert. Not that I'm biased or anything, but if you want a bus that will "go the distance" you're already close to the action and should be able to find a good one with some looking around.
One comment about anybody trying to come into Kalifornia with a "Non-Compliant" (their words not mine) bus/coach or any other vehicle needs to remember that this is all aimed at Commercial/for profit operators and NOT private individuals with whatever funky kind of private NOT for hire vehicles they may choose to wrangle down the highway and byways. They WILL try to Gestapo intimidate them but you simply need to find and carry with you the Kalifornia (DMV? maybe AQMD not sure which agency) regulation documents stating that all the rules apply ONLY to commercial operations. Find this online, print and carry with you. This is especially handy when they try to block entry due to a DD 2-stroke engine in your vehicle. Show them the regs. and invite them to get off your bus, they have no jurisdiction. I've heard of some folks getting turned around for having a perfectly fine DD powered highway coach because they didn't know of these regulations and how they don't apply to private coaches. Remember.....Knowledge IS Power against the tyrants.
We still have the Constitutional right of free travel in our private vehicles, or horses for that matter....ever seen a license requirement or a plate on a Horse?? Neither have I. Any vehicle/conveyance is the same under the law but we've acceded that right over the years, to our everlasting regret. Driving is NOT a privilege, it's our right under the Constitution.