Thermodynamics is the main issue to concern yourself with when you are trying to lose weight. You want to establish an appropriate caloric intake that will create a small deficit in energy in vs. energy out. Once you do this, tissue loss is absolutely going to happen.
However, tissue loss is not the ideal situation. You want fat loss. You will always lose some muscle while in a caloric deficit. This can be minimized with proper macronutrient consumption.
I would worry firstly, about comprising your 1800 calories with enough protein and essential fats. Proteins can be found in lean meats such as turkey, chicken, fish, lean beef, etc. You can also get protein in nuts, cottage cheese, natural peanut butter, protein powders, etc.
Essential fats can be found in things such as nuts, oils, fish, etc.
Once you get this stuff in, the remainder of where your calories come from are not as important for simply weight loss. If you want to optimize your health along with your nutrition, however, I recommend sticking with healthy foods.
Where did you come up with 1800 calories, if you don't mind me asking?