The ability to choose different marketing choices, ie: print media, billboards, sports, sponsorships, product placement in movies/tv, video media, internet media, and how much you spend in each field (obviously, some media types won't be available until they're invented). Also, a way to design your ads would be fun, ie: custom text and a choice of different backgrounds for your vehicle, and to see them appear on the relevent media would be good (like the billboard currently is, but for all media types). And maybe the ability to adjust the percentage of advertising spent on each model of car too.
I guess you could also include/keep the option of setting an amount per region and have it evenly distribute across everything, this would eliminate the micromanagement for those who do not wish to use the feature. History has shown that good advertising will boost business and the lack of it will cause businesses to fail. Inferior products have become more popular than superior products simply because of marketing (VHS vs Beta? C64 vs IBM?). And for your second question, I guess that certain media types would take less money to have a similar marketing effect to another, but if you neglect to advertise in any area, you'll potentially lose out on some business. And as a new media is invented, the popularity of an older media type starts to decline (to a point), therefore creating a need to invest in the new media.
Honestly, I usually just throw some money, randomly spread at all the choices some of the business simulations present to keep everything covered.
However the age and income groups the new simulation will have once it enters the game could work quite nicely with this, as long as the effects of different advertising methods (billboards, newspapers, later radio and TV). However this still needs to remain and quick and easy setup as I don't see players wanting to spent too much time thinking about their ingame advertising.
Maybe just select the advertising type (maybe a tooltip will show you the targeted groups of the method), what you want to advertise (company, or a car) and throw some money at it.
If you could specify exactly how much money you want to spend on a single car, in a single media, in a single region, you could have the highest level of detail possible. In addition to that, because even I think that would be tedious, have options for percentage of advertising to be be spent across all cars, all media and all regions. For instance: Car 1 at 80% focus and Car 2 at 20% focus, just in North America, and evenly spread across all media. You might want something different in another region and change it slightly from that. Or, the same as the previous example, but 80% in newspapers and 20% in billboards and do that for all regions. There could be different buttons that automatically spread the money evenly across all of your cars, all regions or all media types. For people who just want to throw money at it, they can specify a total advertising budget and evenly spread across everything at once. But different regions may demand different cars, or have a higher viewership on one type of media than another, which would make customising your advertising budget worthwhile if you want to maximise your profits while minimising your advertising budget.
Also, the company image should be expanded slightly. There should be image, as in what people think of your comapany (cheap vs expensive, good value vs rip-off, good quality vs bad quality, modern vs old, any other comparison you can think of), and fame/notoriety which is how many people actually know of your company in the first place (this could also go up when you win/enter races etc). You could make the best cars in the world, but if no one knows about them, they won't buy them. If you make the worst cars in the world, it might take a few years or months for people to realise that your cars are not good, but good advertising might persuade them to keep buying anyway. You could also make graphs which show what areas you need improvement in etc.