It depends on if the car is about to enter an expensive period or not, if its not been looked after and/or is an unreliable model then when one thing goes it will be quickly followed by a whole bunch of other repairs. Cars can rapidly become a money pit and sunk cost fallacy applies very strongly to them. Its usually better to be dispassionate and cut your losses, especially if you can make money back breaking the old car yourself.

New vs. old is a difficult one. If you cannot work on the car yourself then having a fixed cost for a car that all non consumables are covered by the dealer can be very attractive. Plus you can offset some of the consumables costs for the first year or two as they will be brand new on the car. If you have the skills, tools, and time to do it yourself then usually its cheaper for a second hand car assuming you can choose a good one thats been looked after. Far too many people who think they know better know fuck all when it comes to choosing a good used car.

This was very much me when I was buying my first car. I was talked into not paying finance on a nearly new car and instead buying my dads old car that had just failed its MoT. I spent a few hundred fixing it myself and getting it back through the MoT. I then did about 50k in it over two or so years but I had to work on that fucker almost every week and spent thousands doing so. With hindsight the couple of hundred on the nearly new car would have worked out about the same cost and I would have saved hundreds of man hours of labor.

I think that there is a lot more mileage in taking a good old car and converting it to be an EV. Its not going to fit every use case but it is going to replace a lot of the old knackered rubbish that is in older ICE cars and is going to cost significantly less to run for the next decade of use before it needs another big refurb. My dream is to get an early Range Rover and do this to it.