Sorry to be a bit of a thread-necro, but there is a lot of mis-information or just outdated information out there
Firstly, the noise itself:
is a vibration, it travels thru any medium (absent in vacuum due to absence of medium). Noise could travel through air (noise), hit a solid object such as metal car body (vibration), pass through it, and keep traveling through air again inside the saloon (noise).
Most of the traditional, old fashioned sound-proofing relies on simply dealing with the very last part of this process - it attempts to dampen the noise inside the car by trying to reflect / absorb sound waves coming off the vibrating car body.
In recent decade there has been major improvement in soundproofing industry, beginning with the realization that to deal with noise - it's just as effective to deal with vibrations of the car body, in fact more so.
Today, soundproofing of a car can obtain as much as 80% noise reduction.
You are correct about the sources of the noise - engine, tires on the road, air rushing past (mostly over the side view mirrors), rain, and other airborne outside (mostly traffic) noise.
Personally, i would not trust small brands - the quality of materials are... just not there, there are massive international brands that deliver the quality, even though may be more expensive than the 'local' brands (i personally just don't like melting rubber in the car)
For example Standartplast (StP) offers a huge variety of materials that deal not only with sound absorption / reflection but a lot more of vibration damping.
Different cars have different issues, some have built-in soundproofing. However none of the currently manufactured cars offer anything that deals with vibrations specifically.
For example, if you are looking to deal with road noise specifically - you have to realize that road noise (from the tires on rough tarmac or gravel) travels into the saloon of the car primarily as vibration. To deal with that issue - you have to deal with vibrations in the wheel arches primarily. There are materials and methods for that specifically, there are 2 types of materials here - some simply dampen vibrations by adding mass, others increase rigidity, which in turn shifts vibrations into higher frequency where it's far easier and more effective to 'kill' them with traditional soundproofing methods.
If you are looking to reduce noise from the rain, which we all know usually completely overrides music volume in the car - there is no point dealing with wheel arches - you have to address the roof of the car primarily, once again there are materials specifically for that purpose.
Engine noise travels into the saloon, once again, primarily via vibrations. Car bonnet should have sound reflecting materials, the firewall needs vibration absorption layered with sound absorption.
One of the things a lot of people do not realize as well - is that a lot of soundproofing materials out there also double up as a heat-absorption materials, which basically means your car is going to be cooler in the wonderful 40 degrees in a traffic jam in the middle of the day.
Sure, doing full soundproofing may not be as cheap as 100 bucks, and will add weight to the car (unless of course you're planning on racing in Singapore), but the level of modern noise reduction (up to 80%) and simple increase in resulting comfort is worth it.