Expansion based on one client is really risky, and can make you vulnerable. Your negotiating position weakens, and if your client decides s/he’s p[aying you too much, what’s your negotiating leverage?
I’d thnk long and hard about taking on the extra overhead, unless you have a contracted workflow ahead.
Maybe it’s time to think about hiring help (freelance, short term, contract, trainee?) with some of the more routine aspects of what you’re doing, so you can put some more time into developing at least one other client?