Hire? Outsource? What are our options?
Hire and outsource. This is sort of simple beginner logic, or? Either the guy doing it is hired from you (employee) or works as separate legal entity (outsource, including freelancers). There is no third way because at the end someone has to do the work.
How to explain this to our customers?
Talk to them.
When we outsource, prices can go high for our customers.
Only if you are greedy bastards. See, there is a market rate for that. THe freelancer will give you some percent and you can mark up some percent and end up with maybe 20% or 25% of the invoice amount in your pockets.
If that is not good enough for you for just forwarding work (note: project management etc. should be separately paid) then you are greedy and yes, i you mark up a lot - you will be too expensive. And no, no sensible freelancer will give you significantly below market rate, why should he?
Should we invest in ads or do it other way?
I really hate to tell you, but you should know. Starting a company is not "i know something", it is totally about doing the paperwork, identifying your markets and having a faint idea how to get people to pay for your services. You sound like a taxi driver who has does not have a driving license. When you consider starting a company, you should have an idea already how to market your services.
Your setup is WEAK. All what you name (exception desktop to a large degree) is heavy graphical, and you have no graphics person. You also have no project management skills. For a pure service company that puts you into the bucket of a body shop, sorry.
I would suggest going passive. I have run a small IT consulting company - nothing big, jut 6 people, and we did not do advertising ever since foundation. There are plenty of companies looking for people for projects.