I have filled our petrol car with diesel - some 20 years ago. Cost £100 to sort out + the £40 for the petrol. And I have done it TWICE. I did it 2 years later too - different car... Cost about the same. Luckily, on both occasions, I realised it just after I had done it, and so didn't drive the car, and cause £500 of damage, like a friend of mine did 3 or 4 years after my second f-ck up!
Also, I'm not being funny, but I am a bit shocked about the tales about the vet bills, in particular the one about an infected kitten they bought from a dealer, and had to spend £8,000 on in vet bills in less than 12 months!
I don't understand why you wouldn't take it back to the seller, and I also don't understand why, if it was that sick, why you wouldn't have the poor thing PTS.
I know this may sound harsh (to some) but I would not be forking out £8,000 on a new kitten I just bought ... If kitty survives, you are going to have a lifetime of high vet bills.
I know someone who was asking for money on gofundme for their dog some time back. He was 3 years old, and had had 7 different operations and procedures in that 3 years, costing £20K. His first at 4 months old! The owners had maxed out their credit cards, and taken out a loan, and had no more access to money, and their dog needed an 8th operation, which was going to cost £5,000. I was like
Not only have they spent MASSIVE amount on him (like £25K+,) but I felt for the dog, being dragged through all of those medical procedures and operations. 
I just can't get my head round it! I mean, I 'get' people wanting to make their cat or dog better, but spending multiple thousands on a pet that you just got, is baffling. As is dragging a pet to the vet time and again, for multiple procedures... forking out five figures for all of the multiple procedures...
JMHO.