I'm looking now to hire someone to run SMM and also someone to do content writing and some other marketing, how do I know who is the right candidate? And should I prefer someone who does it all together or rather find individuals who each of them know their "areas" best?
I'm going to be making some assumptions with the hope that my answer will be helpful to you (if the assumptions are correct) and others.
1. Make sure YOU are "running" all marketing. By this I mean that those you hire should be following your strategy - not creating it. They should be communicating the message you crafted - promoting the offer(s) you created - managing the brand you cultivated.
In other words - don't abdicate marketing strategy to anyone you hire.
2. Depending on where you are in the start-up continuum - pick one in-bound and one out-bound marketing venue. Those will ideally be the lowest investment / highest potential ROI venues based on your market (this presumes you know your CAC and margins). If you are choosing content marketing as one of those venues then run your campaign(s) and keep track of the metrics.
3. Hiring, in all cases, starts best when you know what that position has to do for and in the business to help create value and to increase profits. I like to create a Position Contract that outlines what's required of the position so everyone involved knows what's expected.
With those things in mind - my answer to your question is to hire experts in the specific areas you've identified as the venues you are going to invest in. Get specific and avoid the generalist - at least until you've identified one successful (read "profitable") in-bound and out-bound venue.
Best of luck!