Don't worry about not buying brand new. Second hand cars can be just as reliable, and a hell of a lot cheaper!
I'm definitely no expert, but with over 10 years of driving, I've only had 3 cars. All second hand.
First car was a Nissan. Generally they have good reputation for being reliable (and Japanese cars in general). It was 16 years old, petrol, and had about 70k miles on the clock.
Second car was a 12 year old Peugeot, diesel, think it also had around 70k miles on the clock. I only got rid of the Nissan because family found this car and said it would be better for me as I was now at uni also driving up and down the motorway a lot! French cars tend to get a bad reputation but this car was fine! It was 20 when I sold it for scrap. It had done 110k+ miles. It was only because it failed an MOT and all the fixes weren't worth the cost over buying a new car.
Current car is a Ford, was 14 when I got it. Again diesel, but only 40k miles on the clock. This one should last me years! I think Fords/British cars generally have a good reputation too.
Mileage wise, diesel engines tend to have a longer life than petrol. If I were to buy another petrol I don't think I'd consider anything above 60/70k. I'd be thinking life span of 100k, but this is just a ball park figure, of course they can go for longer. Diesels apparently can go to 200k+ but I'd still be weary of purchasing one above 100k.
Every year with my MOTs I always get a service too.
How many miles you predict you'll do annually would be something to take into account too along with the average lifespans milage wise of petrol vs diesel when you're shopping.
Good luck!