15-20 acres is more than enough for a couple of goats, however they are not as easy to care for as most people think going into it as newbies. They have unique and sometimes complex nutritional needs, they can become mineral or vitamin defficient and that can be very hard to diagnose because it is so subtle. The more acreage they have access to the better, goats want to forage, so they want to eat bushes, trees, vines, anything Not at ground level is preferable. This leads me to the point about parasites and worms, they get these by grazing on the ground in confined areas for too long because the parasites climb up the short blades of grass and wait to be eaten by the goats. When the goats poop them out later onto the grass the cycle continues. The best way to avoid parasites is to give them a large area to roam or rotate from area to area. Give them access to nice green hay at all times so that they can munch on that whenever they want. Think of their stomachs as an engine, it should constantly be fed to keep the gut bacteria working and happy. These prey animals (sheep, cows, goats, etc) sleep maybe a total of 3-4 hours in a 24hr period, the rest of the time they should be digesting or eating. Meat goats do get fat easier than other breeds, so if your intent is for meat sale then I would buy a meat breed. Most goats require some grain to fatten up for butchering, grass, trees and shrubs hold very little calories or fat. If you are feeding neutered male goats however, you must be very careful about feeding too much grain because it can cause a urinary blockage that will kill them within hours. (It is caused by too much phosphorous) If you have access to it, I would feed soaked beet pulp pellets (1 part pellets to 3 parts warm water). Do NOT let beet pulp get wet in storage, it can become dangerous to them, and once soaked, make sure it is eaten within a few hours in warm climates, it can cause acidosis if not careful. Beet pulp is high in calcium, so it helps to balance out the high phosphorous present in grains such as oats and corn.
Be careful when selecting goats to buy, look for a healthy shiny coat, they shouldn't be too skinny and ribby. Check their teeth, by their second season they should have a set of adult teeth but still have baby teeth, look for two different sizes of teeth if the seller says they are still young. Pull back the eyelid and check that the skin under the lid is a nice pink color, not pale. It is safer to buy baby goats in some ways because they have had less exposure to possible diseases. Check the other goats in the seller's herd, be wary for any lumps around the jaw which is a sign of an infectious disease called Caseus Lymphoma. Do not buy goats that have been living in crowded, muddy conditions, the possibility for disease is high. I do not know what country you are in, if it is the USA, Canada, or Australia you can certainly ask the seller if they have ever lab tested for CL and or CAE