Can't offer any advice as regards to what is involved in getting rid and getting a new one after 3 years, but I can help a little with the return standards.
Basically every inspector (if yours is inspected, not every car is, but the inspection numbers are increasing So I would say you are 80% likely to be inspected) works to the BVRLA standards. They can be found on line.
Every car that's inspected will be treated unequally. Despite the fact it shouldn't be it is. As an example..... a Renault Clio with 60k miles on the clock, 3 years old....... has a scuff on the front bumper where it's just caught another car or maybe a post on the corner of a driveway. That scuff could have a local repair (blow in) and it would likely be absolutely fine. This is because it won't overly affect the value of it when it either goes to auction or ends up on the forecourt of a dealer.
Now, a BMW M5 with 5k mikes on it and a year old, that very same scuff will very likely end in the bumper being painted. The reason being, that scuff will affect the value of the car.
In a very basic nutshell......
Imagine your car is sat on the dealers forecourt. What are the things you would get the dealer to sort before you pick it up? They are the things you will likely be billed for.
I would also say, leave any damage on the car unrepaired IF you can deal with it being unsightly. So if you massively curb a wheel after 2 months of ownership and that sort of thing bothers you, have it returned. Just pay particular attention to what the agreement says with regards to wheel returbs. (Diamond cut Vs painted etc)
If the damage is visible and the inspector doesn't have to really investigate why something's been painted, it will save him time and will sometimes be within return standards as far as the inspector/contract goes. Also, even if you get billed for a repair, it is often done at trade prices and not retail, so cheaper that going to a body shop yourself. I would also add, if the repair is done badly, you will have paid for it, and then the inspector could deem it as an unsatisfactory repair which will need work, and often it will end up as a full panel repaint.
Tyres..... make sure you read the contract. It can quite often specify a brand of tyre or a couple of brands of tyre. Bear it in mind for future costs.
What is add to that is, if the tyre is at 1.7mm it's legal. That can be returned no problems.
So if you have illegal tyres say 4 months before it's being returned, there is nothing stopping you having a part work tyre(s) put on. As long as the part work tyre is of the SAME spec (profile speed and load rating) there is no problems. Add to that, if the car has run flat tyres, make sure you got run flat replacements or again you will get charged.
And finally as a side note, when you return the car at the end of the lease you will get an email telling you all of the items to have ready (V5, locking wheel nut, service receipts) make sure you have them out and that the car is clean. It will save the inspector time, and more often that not they will not look quite as hard if the car is bang tidy and accessible.
