figure out what you are comfortable with. figure each time you spend money. how many car payment that would be
for example a $1000 repair to the AC would fund 3 car payments
also have to factor in the down time. Ie will you be renting a car while yours is in the shop.
https://repairpal.com/mazda-mazda3-2008/problems
10 years is a big time frame things like suspension, shocks springs bushings, ect the car will start rattling and creaking a bit more. and so on.. the little plastic bits like knobs and vents will start breaking.
things will start leaking...