you ge started, here are some things you need to consider...
There is colossal amounts of red tape/paperwork. You also have to keep all records for a certain amount of time (6 years if I remember correctly), which requires storage space and a decent filing system.
As an example, the driver's walk round check - MUST be done for each vehicle before the vehicle leaves the yard, and any defect noted could have the potential to have the bus off the road until repaired. This is a legal document and can be requested for inspection by roadside traffic police, VOSA, and the courts.
There is much more, including keeping records of all driver's worked hours, mileage lost due to breakdowns/accidents, keeping records of all routes and timetables (each driver must carry a copy of the timetable for the route they are on and a copy of the fare chart) organising each driver's working week and giving them a copy to have for the week (each driver must also keep a record of the work done in the previous 28 days on them for inspection by VOSA if need be) and then if your route goes over 31 miles, you have to start looking at tachograph regulations. Under 31 route miles comes under GB 'Domestic' driving rules, which are very different to EU (tachograph) rules. They also have complications for when drivers have to switch between both types.
As for vehicles, you really need to know what you are looking at. Bear in mind you will need 3 vehicles to cover a 2 vehicle operation (2 for service, one as standby/maintenance rotation). Sub £10k, you're looking at 12+ years old, which means lots of variety, but some real moneypits that need avoiding. Avoid anything that is rare - as an example a previous company I worked for bought 3 Neoplan N4016 Low-Floor buses from Arriva in Liverpool very cheap. They were cheap because they'd spent 16 years being thrashed around Liverpool city centre and also because they were the last of their type in existence. Consequently, getting parts for them has almost always ended up with the part being made to order, or adapted from something else as secondhand parts don't exist. On the other hand, a Volvo B10M or B10B (M = mid-engined, B = rear engined) is a doddle to get parts for due to the sheer amount of the things built and still in service, as well as good support from Volvo.
A chassis may also have different engine options - You might be looking at a Leyland Olympian decker for example, but it could have Gardner, Cummins or Leyland engines fitted, along with either a ZF, Voith or Leyland gearbox.
It's not just the chassis, you need to consider the body - many buses are built as a chassis and then shipped off to whoever is the buyer's preferred supplier of bodywork. If your bus has a common chassis but a rare body you'll still have a struggle to get bits. Bodies by Alexander, Plaxton, Northern Counties and East Lancs Coachworks are common, though I don't rate the ELC bodies.
Still, I digress. Fuel economy is usually anywhere around the 6-11 mpg area. Servicing isn't like a car, a PSV must be inspected every 4 weeks (6 weeks in certain circumstances) and the inspection is basically an MoT without the certificate. Written records must be kept of each inspection and the inspection must be made by a qualified mechanic. Oil changes and the like are as dictated by the manufacturer. Bear in mind that if you have a bus that covers around 900-1000 miles a week (rough average of 150 miles a day) then it'll probably require an oil change every 8 weeks. Wear and tear is quite heavy on most buses, bushes, tyres, brake shoes/drums and alternator belts being the main consumables. On the subject of tyres, they cost (for a 275/70/22.5) anywhere between £250 for a budget and £400+ for something like a Michelin. However they can be re-cut a further 3mm once they are worn down to 1mm (different rules for PSV tyres).
As a rough guide, here's the operating costs of a route local to me, that uses a pair of Mercedes 27-seat minibuses:
Fuel: £700 per week (both vehicles)
Driver's wages: £1256 - 165 hours @ £7.60 p/h (3 drivers working 55hr/6 day weeks)
Mechanics time: £40-50 - 4-5hours per week, though this is averaged down from monthly vehicle downtime
VED: £40 per month (both vehicles)
Total: £2,046 per week, without depot costs, parts costs, even silly stuff like ticket rolls, antifreeze, oil, bulbs, etc.
On top of this, you also have drivers to keep. You must provide 35 hours training every five years (normally through an approved provider) to keep their CPC valid. Whilst it is not a legal requirement for the operator to provide and organise the training, you will find that your drivers will simply move away to a company that will provide it, which is practically every company in the UK. It is your responsibility to make sure they do not drive over their maximum hours, and to ensure breaks and rest periods are observed - the penalties for ignoring these laws can go as far as removing your O-licence, which is also something VOSA can do if you are repeatedly stopped and found to be running defective vehicles (covered by prohibition orders/PG9's).
This of course is before we have even started to scratch the surface of whether you can actually compete on your chosen route and turn a profit.
On the subject of maintenance, sadly many big bus companies pay their engineering managers by giving them bonuses for not using the whole maintenance budget - basically, there is no incentive to spend money maintaining a bus any further than the bare minimum. Quite the opposite, in fact.