Upfront installation makes sense
It's a lot more convenient to buy accessories at the factory-fitting stage rather than after you have taken delivery of your new car.
There will usually be a waiting period before your car is ready, so it makes sense to organise the fitting of any extras at this stage. That way your car won't need to be off the road if you decide to have certain items fitted down the track. Some manufacturers bundle different accessory packages together and make them available only when the car is sold, so be sure to check with your dealer.
You'll also need to find out from your dealer which extras, if any, are included in your overall purchase price. Then you can discuss what else is available with your chosen vehicle. You may be able to negotiate a better price for some accessories at this point, but this will differ from dealer to dealer.
Talk to them about what's available and be upfront about what your needs are now - and whether they might change during the period you plan to keep the car.
The dealer has the expertise to guide you through your options. In the end, it will be entirely your decision as to what suits your situation and budget.
Rather than buying no-name parts through an auto-store, it's a good idea to buy 'genuine accessories' directly from the car's manufacturer, even with used cars. That way you can have greater assurance that the part will do its job properly.
Accessories that are specifically designed for your car will look better, fit well and perform exactly as they were designed to.
For example, if you buy a 'genuine Toyota accessory' for your Toyota car, you will know it's been designed and manufactured to comply with Toyota's stringent engineering and testing standards.
Things to consider
It's important to remember that while some accessories, such as alloy wheels or changes to the vehicle's body can add value to your car, they may also affect your car insurance premium. Check with your insurer before you make a final decision.
If you've bought a second-hand car, it might not always be possible to post-fit the exact accessories you want. Check with the car's manufacturer or visit a local dealer.