99.9% of the time, when we talk about eating our midday meal, we say 'have lunch'. I have lunch in the canteen. Do you want to have lunch together tomorrow? I'm hungry because I haven't had lunch yet. This is the same for all meals - breakfast, lunch, dinner, supper. The only time when you would say 'a lunch' is when this is a formal occasion or celebration of some kind. For example, you might say: 'We're going to hold a special meal for the CEO's retirement and invite all the company's employees. We'll either have a lunch or a dinner at a nearby hotel.' But, as you can see, this is a fairly unusual usage.

I would add to this that, for me, there is two options in this situation. 1) Let's have lunch! OR, quite commonly: 2) Let's have some lunch! I would never really use "a lunch" in this context. It is more to describe a formal ocassion that you will attend, for example: "I have been invited to a lunch at the club with Sarah's friends". It refers to the ocassion, and not JUST the meal. Tom