1. I have had good success buying jerseys and shorts off of eBay. You just have to know your sizes and exactly what you are buying.
2. It depends on how often you plan on washing them. If you wash them once a week then you need enough sets to last one week.
3. I wash them in the washer after they are used. I never put on a dirty set. A dirty set is more likely to chaff and it is more likely to have bacteria in it. NEVER let them go through the drier. Take them out of the washer and hang them up to dry. You can also hand wash them in the sink if you are only washing a couple sets.
Also, keep in mind there are two types of soreness that you get in your nether regions from riding: chaffing and pressure. Chaffing is cured by using chamois cream. Some symptoms of chaffing are rashes, raw spots and scabs. Pressure relief is more complicated requiring proper positioning of the saddle, the correct pad for your body and the correct saddle for your body. Some symptoms of pressure issues are pain, and tingling/numbness in your bottom and/or your feet. Heavier riders tend to benefit more from thicker gel pads more than the lighter riders. I am riding with a 2 cm gel pad until I lose 35 pounds. It has made a world of difference for me compared to a standard 12 mm pad.