As an owner of two classic buses, I hope I have a contribution here...
As has already been said, buying is the easy part! Storage is the killer. Outside storage is both easier to find and cheaper than inside storage; but it's a false economy. Buses kept outside deteriorate much faster than a car because of the way they're built, with lots of seams and gaps for moisture to get into, with big holes such as gaps under entrance doors (or for a rear loader, no door at all). Most indoor storage is in farm barns, check that there's a concrete floor, lighting and power. Cost varies hugely around the country. London & South East is dearest but expect to pay at least £75 a month no matter where you are. Most people are bright enough to check that the barn door is high enough to take a bus, but what about sharp bends and low trees on the lane to get there?
Cost of buying varies enormously. This is for several reasons:
1. There's always a premium for London buses.
2. There's a premium for buses that look the part for wedding hire and maybe have a Class VI ticket.
3. The market gets skewed by buses offered for sale by commercial companies as bars, conversions etc or sold by companies who bought a bus for a commercial purpose, changed their minds but still want top dollar.
4. It's not like Glass's Guide: there's not really a very well-defined 'going rate' for a model/year because the volume of sales is so low. Putting the commercial guys aside, people buy the bus that appeals to them because they went to school on it etc; and in those circumstances the price simply depends on how badly they want it.
5. And yes, a rot box will cost less than a fully-restored bus.
If you're handy then you can do a lot of the restoration yourself; but everything takes far longer than you expect, as I found out this weekend when stripping paint from the interior of one of mine. The phrase 'two steps forward, one step back' could have been created for bus restoration. For bought-in services expect to pay £50-75 per hour for labour, plus materials. For work you're doing yourself, a mate/assistant is virtually essential: partly because so many components are too heavy to lift compared to a car, and partly - and I don't want to exaggerate - it can be dangerous and you need someone to watch and be available to summon assistance.
Spare parts availability varies hugely depending on what you buy. Early Dennis Dart SLF? Not too bad, and lots of them in the Barnsley scrapyards. Crossley DD42? Get saving up to have things re-engineered or made from scratch. Ebay and bus preservation Facebook groups are your friends but your network of contacts is indispensible.
Pretty much all buses had a service and maintenance manual (can normally get on Ebay) but they're not for the beginner, forget the Haynes approach. My friends and I used to joke that the manual's instructions to remove a Leyland PD1 engine went something like this:
"1. Turn off engine, apply handbrake, disconnect battery terminals.
2. Remove engine.
3. Installation of the engine is the reverse of the above procedure."
A joke description, but only just! So either learn to be handy or pay a lot for restoration and maintenance.
Take out a subscription to Bus & Coach Preservation magazine, partly for hints & tips and partly for the classified ads.
For driving, if the vehicle is over 40 years old then you can tax and drive on a car licence so long as you don't carry more than 8 passengers. If the bus is less than 40 years old or you want to carry more than 8 people, you need a PCV licence (you don't need the additional CPC if it's not for hire & reward).
Insurance from specialist providers is surprisingly cheap, this is because the specialists know that preserved buses don't do many miles and are treated gently on the road. For two buses I'm paying £350 a year but it will vary depending on age, location, how many years you've had a licence etc. Don't bother with a general car insurance company.
To answer your question you can own a bus and insure others to drive it. Registering and insuring it works just like it does with a car, you get a V5. Buses over 40 years old are exempt from MOT class V and tax is free (historic vehicle) but many if not most owners including me submit to an MOT test anyway, for peace of mind.
If all the above seems intimidating, it is. Owning a bus isn't a cheap hobby and for every bus that you see at a rally, there was another stalled project that ended at the scrap merchant. The Bristol RE I'm restoring now cost me £2,000 to buy; but so far restoration has already cost more than that (the radiator rebuild alone cost £1,100) and next year I'm planning to have the seats reupholstered at a cost of about £4,000 (and that's even after I bought a roll of moquette for £500).
But once you're done, there's no better feeling than driving down the road with the windows open, getting waves from kids, trundling along the A roads.
Feel free to PM me if you have any specific questions or have your eye on a bus.
My fleet, December 2018 by
Paul Williams, on Flickr