Welcome! You will enjoy this site, if you are interested in cattle.
#1. First of all, I do not live in your area, so my prices may well be far from appropriate for you. In this area, calf prices vary greatly. You can get Jersey bull calves and Jersey crosses for free. Holstein bull calves don't bring much either, day olds are usually less than $50. Colored beef calves can bring up to $100, if they are very nice. You will be disappointed if you buy a calf that is not in "great shape". Calves that have been exposed to sale barn conditions have plenty of things that can (and will) cause them to die, and die quickly. In my opinion, you should buy a calf in the best shape possible, and get it from the farm/ranch where it was born. This will limit the possibility of exposure to the many viruses and bacteria that are rampant in sale barns.
#2. Yes, many dairies routinely sell their excess bull calves. Some beef feedlot owners also have heifers that calve, and they sell these calves also. These are usually a day or two old, and they require milk replacer for up to 2 months, till they are well started on feed. Cost? Around $70 - $100 for the milk replacer, and probably $25 for the calf starter. Do NOT buy the cheaper milk replacer. It likely will cost you far more in decreased growth, and you may possibly lose your calf.
#3. Yes, weaned calves should have "feed". Their digestive systems are not developed enough to allow them to grow and maintain themselves. Calves that are on "range" or "pasture" can depend on their mothers milk for up to 7 - 8 months, while they are eating grass. The calves that are weaned off milk at 2 months are not able to survive without a high energy, high nutrient density diet. There are dozens of brands that are excellent quality feeds, and you could also formulate your own from locally grown grains.
#4. First of all, don't quit your day job. The profit margin is extremely slim in the cattle business. Be sure to have all the equipment, feed, and necessary shelter on hand and ready to go before you acquire your calf or calves. Establish a relationship with a large animal veterinarian. Plan to castrate a bull calf, and if your calf has horns, be sure to have them removed. Cattle can easily maim or kill humans, even when the cattle are dehorned. With horns, cattle are as dangerous as drunk drivers. They can seriously hurt or kill you when they are just playing. I cannot stress this enough. Dehorn, dehorn, dehorn! The life you save may be your own or your loved ones.
At any rate, I wish you the best of luck if you should go ahead with your plans.