I started my career as an engineer and transitioned into advertising and marketing.
Looking back, one thing I would have done is go and work for a successful ad/marketing agency, before starting my own company. Even if I had to work for free.

This would have shortened my learning curve regarding getting clients, delivering work, proposals and how to base your fees.

I got to where I needed to get, but took much longer.

When you say "We only need it for SEO". That's what Google doesn't want.
Directories do provide value other than SEO, if you can find the directories that actually provide a service for your industry, like organizing and ranking suppliers for a specific niche. Then those directories do provide.

Using a bulk directory submission strategy is a waste of time, money and could be considered spammy.

If your site is focused on providing great value for a particular audience, then your website should have great content and a good website structure that is user friendly. If this is the case, then search engines will find your website and include your information in their search results.

I would spend some money on an SEO consultant to make sure the structure of your website is optimized for search engines.

A PPC campaign doesn't make sense if you don't have specific keyword to target, unless using a display campaign.

I have many different sites that don't focus on one particular keyword, these sites receive thousands of search engine referals from all sorts of search terms.
My sites include directories, news publications, product catalogs and informational websites. For all these websites Organic search is the number 1 referrer of traffic.

So, yes SEO is a good choice, but you may not need a continuos campaign, just someone to get the structure correct in the beginning.

The scope really needs to be more defined. The website alone for 50+ dealerships is a major undertaking. There are so many questions that need to be asked and answered before you even start proposing solutions...
I have an agency friend who worked on developing a new website for an auto delearship, the budget was in the Millions of dollars.