With Contract Hire it is very simple as you are basically 'renting' the vehicle for an agreed period of time. Usually you pay 3 months rent up front and then monthly afterwards. You have an agreed mileage allowance a year (10,-15,000) and anything over that is charged per mile usually about £30-50 per 1000 miles over that at the end of the agreement.
You are usually allowed to fix things (systems, ply-lining etc.) to the vehicles as long as it does not damage (nothing that a rubber bung will not fix) or will not affect re-sale. You are allowed usual wear and tear including minor scratches and scuffs, but actual damage will have to be fixed. At the end of the contract you can simply hand the vehicle back and pay nothing at all, some firms will then offer you the vehicle to purchase at a discounted price.
The great thing about CH is that the cost is treated as a 100% expense. If you take out a maintenance contract along with the vehicle then this is also 100% deductible - usually about £30 a month for all servicing/tyres/exhausts/faults/etc.