What you need to change no matter what

So, you've already been to the garage and have in hand a list of what needs replacing on the car.

Find a reliable supplier of parts for your car (see the article on how to choose suspension parts), and also pick out the right engine oil that you'll be using in this engine going forward (see the corresponding article on choosing engine oil). After that you can order parts as this set:

• Everything the garage listed for the suspension (even the items filed under “replace soon”), plus everything that “surfaced” during the electronics diagnostic check.
• Engine oil in a quantity that exceeds what you actually need by at least half a litre.
• An oil filter (don't even think about saving money here — buy the most expensive one available).
• Fuel and air filters (same rule as with the oil filter).
• All drive belts: the toothed timing belt, the alternator belt, the air conditioning belt, the high-pressure fuel pump belt (for diesel engines), and so on, plus all the tensioners and pulleys whose replacement is specified alongside the belts. The best way to find out exactly which belts, tensioners and pulleys need replacing on your particular engine is to consult an official brand representative or check the service book (if it has that information). It's worth stressing that belts and pulleys are best bought as genuine parts from the manufacturer — these parts get replaced once every several years, and the labour to replace them is a fairly expensive and lengthy job, so cutting corners here can lead to significant extra costs down the road.
• Crankshaft and camshaft seals (even if no leak was found during inspection). When replacing the belts, it's best to replace these seals regardless of their condition — remember that for the next 60,000-100,000 km you'd rather not go back into that corner of the engine bay. Buy only genuine seals — they're not expensive enough to be worth saving on, and an unscheduled replacement later will cost far more.
• If your engine has several camshafts connected by a chain, be sure to check with an experienced mechanic whether it's time to replace the chain.
• The water pump (the cooling system pump). This unit is also usually only replaced with all the belts removed, and besides, most manufacturers build a regular pump replacement into their service schedule — and given how little reliable information you have about the car, it's better to replace it. It's also best to buy a genuine pump, given how important the unit is and how expensive the replacement labour is.
• Engine mounts and the subframe. In most cases these rubber components are practically impossible to diagnose while the car is fully assembled, but their condition has a very significant effect on driving safety and on the service life of every part of the suspension. If possible, just set them aside at the shop (or buy them with a return option), so that if needed you can quickly bring them to the garage while the car is still disassembled and other work is underway.
• All service fluids: power steering fluid, brake fluid, coolant/antifreeze (only use what the car manufacturer specifies — these fluids are not interchangeable!). These fluids need replacing every two or three years (depending on the make and model), because fluids lose their service properties over time and mileage. What's more, by default you don't know exactly what's in your car — it's entirely possible the cooling system just has water in it, or a very weakly concentrated solution, which in any case will wreck the cooling system at the first hard frost.
• If the car has an automatic transmission (AT) — you need to replace the transmission fluid, along with the filter and the pan gasket (if the fluid change is done with the pan removed). Very important: if a genuine transmission fluid exists from the car manufacturer for your transmission, use only that! Repairing an automatic transmission is incomparably more expensive than the price difference between the genuine transmission fluid and a substitute. This recommendation holds even when the substitute is offered by world-famous brands — automatic transmissions are extremely sensitive to oil quality, and besides, in most cases every car manufacturer has its own requirements for the composition and characteristics of this fluid.
• If the car has transfer cases and a driveshaft (mainly rear- and all-wheel-drive cars), you need to replace all the oils in these units, as well as the gaskets and seals whose replacement is specified along with the service fluid change.