Hey - I have been in creative client services for most of my career, ranging from running global strategy for fortune 500 companies to building websites for tiny non-profits. Hope these thoughts are helpful:

Use freelancers as much as possible until you get your feet under you. The main reason: no fixed costs to drain your cash during down months.

That being said, try to get one partner/hire early on who can pitch in across a number of categories - marketing, design, development, project management. It helps having a friend, and having another committed person can help scale/stretch drastically.

Present as a cohesive company, but don't be afraid to expose your financials to clients. This has been a selling point for me: "World class talent without paying for overhead" is attractive to clients.

Do fixed fee 99% of the time. No hourly.

Process is everything. All these services are so commoditized these days, that great customer service and a clear process can go a long way.

I would be in charge of the client relationship almost all of the time if you can. But, I would need to know more about your company. Generally speaking, relationships are the most valuable asset you have in creative services.

Hope that helps!