If you want a new car then it makes sense not to spend any more on your present one.

If you however like your present car and are happy to keep another few years then the parts you require are only consumables that any and every car needs, though i qustion the cost involved, unless you have a T4 £300 at most will buy you a set of high quality boots and another £300 (£400 tops) should redisk and repad it all round.

Your mileage and driving pattern may not suit a modern Diesel car, Xantia generation Diesels were superb (similar Pug 405's used to run 250k regularly with just servicing), things have moved on (backwards) since then, fast and frugal they now are, but horrendously complex now and not as robust, when they go wrong the costs can be eye watering.

Your 1k is a little bit on the top side in fact its a lot on the top side half it and you will be somewhere near.

At 8k a year the last thing you want is a HDI in fact you do not want a diesel full stop maybe C4 petrol would be a much better bet.

As the car is worth less than £4k, I have to decide whether or not to spend the cash and keep it, or change it.

Tyres and brakes are consumables. If your quandry is an economic one, you need only look at absolute costs. The cost of renewing tyres and brakes bears no relation to the value of the car. If you're looking to change, you need not concern yourself with the maintainance costys of your current car.