1. Advertising is perhaps one of the most important aspects of selling a car. However, it's difficult to make the mechanic function in a way that's fun. It usually comes down to, "how much do you want to spend on X" -- and that's it. I don't know how difficult it would be to implement, but I envision something along the lines of importing a background picture into the game, and being able to overlay a photo of the car you designed. Then, you can add text via a text box and make (and save) your own ads in game. Each ad you create is the basis for an ad "campaign". That is, you choose how long you want the campaign to run for (which will determine cost), the target audience (which will determine the effectiveness) etc. I think that would be much more engaging than, "TV ad = $X amount = Y sales boost". Although if the latter means the game is released sooner, I'll take it!
2. Why should everyone start with a small factory? Suppose I want to recreate an actual car company that was already well-established, well-funded, and making cars in 1946. I think the game should allow for a variety of starting scenarios.
3. That's seems to be a given, IMO.
4. Again, why the arbitrary limit? I envision something more along the lines of franchising agreements. Various AI dealers compete for your business. Depending on the size and location of the dealer network, they will offer your company franchise fees (and ongoing royalties) for selling your car. The revenue from which can be used to advertise or build more cars.

So for instance, AI dealer #2948 will pay $50,000 for the right to sell your cars and $5,000/year for 5 years. If the cars the dealer buys from your company fly off the lot, not only will you have more dealers competing for franchise agreements (and you can have as many as you want), the terms will become increasingly favorable. On the other hand, if the dealer ends up with a crap car that has an average 500 days turnover, you'll have a hard time getting franchise agreements and the ones that do come along will be very small. The easiest way (and the one that would probably take the shortest amount of development time) would be to bypass the dealer mechanic entirely, and do as Tesla does. Direct sales.
