The receivers today feature a lot of applications, so you must know the purpose for which you will use it. In principle, as basic integrated features, the unit should have a Bluetooth connection, USB and AUX connections, a larger display for easier use and even a navigation system if you like to travel or you live in a crowded town.
It would be better to consult closely and try it beforehand, because even if it's a well-known brand it can be a very bad product.
Also, before choosing a receiver you must consult the market to choose a head unit that will be useful for you and if it will fit in your car center console because there are two kinds of receivers: Single DIN and Double DIN.
If you have space for a 2DIN you can install even a 1DIN with the aid of a bracket. If you have space for a 1DIN you can choose a receiver with a folding screen or you can customize your center console.
If you plan to buy an amplifier you need to know what you will use it for, the type of music and how much listening you plan on doing.
The amplifier should be a class D because it has a lower consumption but can develop more power and it should be a MOSFET amplifier (Metal Oxide Semiconductor Field Effect Transistors). Attention, maximum power is not necessarily a term of comparison for performance as a most people think.
Before choosing some new speakers, you must know that the power is measured in dB (decibels) and not in Watts and it would be good to have SPL (sound pressure level) specifications. Also you should know how much RMS power it develops.
It would be better to look at the materials used to create the speaker because if it's built from high quality materials, it will stand the test of time and it will produce a high quality sound. You must know what size of speakers fits in and you shouldn't be surprised if they don't fit in the door racks.
If you plan on including a sub-woofer in your car audio design, you should know exactly where it will go beforehand.
This way you will avoid unwanted surprises. Also, it's a good idea to get informed about the power it develops in dB and Watts before you go on to purchasing the actual unit.
You will need to buy some cable kits which you can find in any specialized shop.
Remember that once you upgrade your audio system, it is likely that you'll need a larger battery, or an additional one, due to the fact that such a system consumes way more energy than standard audio systems do.
My advice is to do nothing in a hurry because most likely you'll screw it up. Some common mistakes which people do when installing a new car audio system include poor understanding of the cabling system, where each cable goes and what it powers.
This can mean extra work since you'll have to dismantle everything again and some of the car parts could get damaged. If this is the case, you'll run into some nasty unexpected costs which can climb pretty high if your car is a rare or outdated model.