Not sure how much help this will be but, for what it's worth, here's some general things I noticed about this level:
The biggest time constraint I ran into was not having very many sports cars go by mid to later in the level. I'd have the cash ready to go, but nothing to buy!!!
It seems like most of the more reasonably priced (under 170,000) ones come by early in the level . . .
With that in mind, I've sold off one of the cabrios immediately, as is, upgrading one other for sale (just tuning or engine too if time permits). That way, I can get a big lump of money early on to buy a sports car. I sell off a second cabrio right away as well to have cash ready. I only buy other cabrios if they're a steal during this time period.
When you sell the second cabrio, try to hold out for a green thumb price if you can.
You should be able to get the four sports cars pretty quickly this way. The next hurdle is getting the million . . .
I bought and upgraded any cabrio that was a decent price (under list but not necessarily green thumb), selling them off as quickly as possible.
You'll also need to have 11 or 12 employees, so hire batches of 3 as you can to be ready.
For some reason, I think all of your employees have to be idle when you get a million in order to click the green check to "train" the employees to fix sports cars so keep that in mind as you choose to sell off cars--make sure no one's tied up selling for too long.
Once you can fix sports cars, you'll need to buy a bunch of parts so be prepared for that as well . . .
The final hurdle is making rent since the sports cars alone won't do it. The good thing is that you probably have a ton of cash so you can just buy random working cars to fill in the lot if you have to at the end. If you haven't sold off all of the cabrios you collected, you'll hit the 120k rent easily.
There it is, a bunch of disjointed random "tips". Hope at least some of it helps!!