Hi, as a former client services director for a dutch software agency I think I can help you with this.
1. Always stay in the lead if it comes to communication. Make sure you know what the customer wants and make sure your agency is the middle man that gives the orders to the developers. This is not only the best way to keep the business, but you also guard your end succes with this involvement and thus keeping the customer happy. Make sure you, or your colleague is the first person the customer calls if they have a (software related) problem by solving them!
2. If your company is the responsible party in the SLA then you will just need to mention that you work with external developers, there is no need to mention other company names if your company is responsible for the end result / if things go wrong.
3. In a word: communication. So many projects have gone wrong because of high expectations, no expectation management and poor realization. If you only have introvert people working for your company (mainly developer types), you would probably do good by getting someone who can speak up to the client and the developers, a communicative expert who can support you in guiding the project towards the customers satisfaction.
4. Just make sure you over deliver by doing the aforementioned really, really well. For example, plan a MoSCoW session before the project start. Start with an MVP first. And make sure you do Scrum meetings so you keep close watch on the project.
If you have any follow up questions, feel free to plan a call.