I have covered something similar to this in the past, what post it was now, I don't remember so I'll just write it again...
Firstly, let us consider the speed difference between a high end super car that reaches your 120 MPH threshold and a low-mid range car. So in this case on the high end I will use the Mercedes SLS AMG (SLS) which is the first car which can reach 120 MPH with no upgrades. For the low tier I will use a Honda S2000 (S2000) a vehicle I think we can all agree should go faster than it actually does at only 46 MPH base. That gives us 120 - 46 = 74 MPH to make up for in upgrades, that means the S2000 will need to go nearly triple the speed it does originally and with upgrades only giving around 8 MPH increases on average for each one, we need to have just about 9 total upgrades available for it. Each upgrade is composed of 5 different script files, so that means 45 additional files just for the S2000 alone, not to mention each one has to be changed and tested individually to ensure it is not broken or uncontrollable (which vehicles easily are) so let's assume 30 seconds per as some go faster than others, but there's the original set up as well. 45 * 30 = 1,350 seconds / 60 seconds = 22.5 minutes for just the one vehicle. Now we would need to do this to all the possible cars, so we have over 100+ vehicles in that case, so that's then 22.5 * 100 = 2,250 minutes / 60 minutes = 37.5 hours and then some, as it's more like 120+ cars so add on around 8 more hours.
However, we're now missing an important factor in all of this, price, which was the main determining factor in how cars are balanced right now. With our S2000 example, it needed 9 upgrades, and each upgrade for it costs $150,000 so... 9 * $150,000 = $1,350,000 and it has a base cost of $295,000 therefore the total cost for it to go 120 MPH would be around $1,645,000 while our SLS which did 120 MPH base is $3,220,000 effectively double the resulting cost of the S2000's required upgrades. Of course it may be different if we scale the upgrade cost based off the number of upgrades already on a vehicle, however this would then negatively effect all other balancing of current upgrades so massive changes and logistics would need to go into that compounding the time required immensely. Of course there is an element of prestige that comes with owning an SLS over an S2000, however double the price? That seems a bit of a stretch.
This all being said though, I fully understand the desire and idea behind this as I've had ideas of how we might be able to allow for some extra cars that allow for faster versions without killing all the balancing, but anything with cars is very time consuming when originally setting up and then later on whenever we need to change things regarding them. The biggest issue is that people own so many cars right now that if we shuffle any of it around, some people effectively would gain lots of money off it and others would lose lots of money, so it's not something we can tweak very easily.
A big issue we face is the size of a map that Source allows for, as many people may note some of the faster vehicles are a bit impractical because of as soon as they actually get to max speed, the next turn on the highway happens which reduces your speed greatly. Because of this, we can't have major differences the lowest tier car to the highest tier car, practically speaking it is only around 100 MPH difference, excluding any upgrades.
TL;DR
It is a ton of work to even attempt making this many new variants, but there may be some changes in the future to have faster options of certain vehicles that should have faster ones, unsure at this time. As for now, this is too excessive.
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