You can pretty much do all of the things you asked. The equipment you need will differ, depending which way you decide to go. If I were you I would go with a new H/U. The stock monsoon H/U is not a good way to start building a system. Some people like them for the stock look or are afraid of theft, but an aftermarket H/U will give you better sound quality, and more control to adjust things to your liking. A new head unit will install easily and be a good foundation for building the rest of your system.
If you decide to get one just make sure the unit has all the options you are looking for like 3 sets of RCAs out, built in crossovers, aux inputs, stuff like that. A unit with these features will be a bit more expensive up front, but down the road it will make your upgrades easier and you will be happier that you didn't screw yourself in the begining. Good luck.