Faced the same ‘dilemma’ when I came out of a Company car company a few years ago, Company cars are expensive in tax but hassle free, literally nothing to think about, a allowance is good in that if you use it to finance a car you will eventually own it with the company having paid for it, I did as you have suggested and got a 4-5 year old car for 10k and the allowance plus mileage claim made it cost neutral over 5 years, I still have the car, all paid for but it’s starship mileage so not worth much now, you could lease with the allowance but you are tied to the lease if the job doesn’t work out, that’s what stopped me doing it.