meets1: I understand your situation. When I was in the landscape maintenance business, I tried to do as much as I could for a given client. But I only wanted to provide them with the services I really knew well and had prepared myself for. As an example, I learned irrigation maintenance and also obtained a pesticide applicator's license so that I could provide those services in addition to the less-technical mowing, trimming, and mulching. But I avoided any projects that I didn't know well. That's the basis for my advice to you.
If you don't know enough about low-voltage lighting to even know where to start, don't start. Advise your client that even though you want to serve them, this is an area that you don't have the experience or know-how. I think they will appreciate you for that admission.
Starry Night Lighting
"This little light of mine, I'm gonna let it shine."