That is a great thing to make sure you have in your business plan, that your business will be built around those real customer wants. Do not make callous assumptions, or statements like "I know what people want", "People are going to love this", and so on. Have you done your research? Do you really know that the people you will be targeting want your product / service, and crucially do they want it AT THE PRICE that you will be offering it at? Whilst confidence in your plan is fantastic, you must make sure that it does not lead you down a blind alley along a path that is not desired by your target market. Don't assume what customers want, do your research and make sure that is clear from the start in your business plan.
Variable expenses are those expenses that track directly with sales. If sales stop they stop. These are expenses like supplies used to support in the making of your product or doing your service. Such things as shipping cost for raw materials for your product or service. If you have no sales then you're not going to be purchasing materials so your shipping cost for those materials will stop as well. As an example, if you have a lawn mowing business and there are no lawns to mow, then you wouldn't be buying gasoline to travel to your lawn mowing site. These kinds of things are variable expenses. If you're producing a product, it would include supplies used to produce that product like sand paper, glue, finishing materials, cutting tools, etc.