Finally I can post without getting arrows in my chest!
As a marketer, and given the tiny little bit of info you've given, you can probably afford one person on staff, or an agency part time in your local city.
Just be aware, and this is the part that sucks, you need to make sure you have:
a) A clear idea of the consumer's problem you're trying to solve.
b) A great solution for that solves it.
Failure to have either one of those will lead to you chasing a bad product into irrelevance. And it's really expensive.
How do you make sure they're accountable, well, that'll be defined by what you and the marketing person agree is your metric for success. Is it sales? Target market penetration? Site Traffic? Word of Mouth/google trends? All of these goals will be treated differently through the eyes of the marketing person, and you need to make sure which one you want. There are strengths and weaknesses to all of them.
If you're concerned about bringing in non-performers, then do NOT enter in a "partnership" with them. Hire them outright. That way you can fire them if they don't meet the agreed upon metrics. Trust me, there are many, many charlatans out there who will gladly take your money and give you next to nothing in return. Conversely, one failed campaign is not an indicator of non-performance. It may take a few stabs to figure out what it takes to make people want to buy your product, but consistent underperformance is the sign of bad marketing. Or a bad product.
You cannot discount the idea that you may have a bad product. And nothing, and I mean nothing, will kill a bad product faster than great marketing. Once you get people's interest, you had better have a product that lives up to the marketing promise or word of mouth will sink you faster than the Titanic. Seriously, take a page out of Apple's marketing book: Underpromise, overdeliver. You don't have to overdeliver by much, but over deliver.
There's a lot more, but that should be enough to get you started.