What is the most effective way that a start-up can go about hiring a marketing firm?

Tim, you are rolling two different aspects into the same question, namely sales and marketing.

The basic distinction is that sales involves taking an existing product and getting people to buy it. McDonalds sells hamburgers and Coke sells soda. Sales are made one at a time to an individual customer.

Marketing is what comes before sales, it involves working with potential customers to understand their needs, develop solutions (products) and come up with pricing strategies. Marketing generally does not deal with individual customers but rather with broad markets, brand management, strategy etc.

Unfortunately in our modern society the language often becomes corrupted. The garbage man becomes a sanitation engineer and the used car salesman becomes a marketing professional.

It makes perfect sense to pay salespeople on commission, after all it is their job to make sales. It really does not apply so nicely to a firm that will help you develop a marketing strategy. You state:

We have our pricing models complete but it's a best guess based on competition and we know the target market but we are not marketers so, we thought that we should go with a marketing firm that specializes in marketing IT related services.

Perhaps your first step is to bring in a marketing consultant for a fixed fee to review your current plan and possibly help you improve it.

Once you have a marketing plan you feel comfortable with then hire salespersons on commission (or whatever basis appropriate).

You might want to look into direct marketing agencies. A few of these will agree to "pay for performance" agreements with the condition that you shoulder all of the expenses.

One of the reasons why I recommend direct marketing is that these agencies know which elements of the campaign to test before you put a lot of money behind it.