A water filtration system really isn't necessary. The system will purify the calcium deposits that are found in water. This can be a good investment if your car has high deposits of calcium but if not it really isn't necessary. Calcium can lead to hard water buildup that lead to etching in the clear coat of your paint overtime. However, as long as necessary precautions are taken care of such as drying your car after washing, which you should do, your car will not experience these deposits. Polishing will also take care of removing any hard water spots. If you begin to notice hard water spot buildup after drying your car then it may be a good idea to get a filtration system. If not, don't worry about it.
If you are looking for the extra depth of your paint from the souveran then I would definetely go for it. As far as maintaining your car I would be very conscious about washing the car carefully. Always use the two bucket system to help minimize further swirling. On the other hand I stress the importance not to fully detail your car every month. Machine polishing removes imperfections by cutting your clear coat. If the paint is repetitively polishing, as in every month then the clear coat will be cut away. I would only fully detail your car every 6 months. If swirls and scratches do begin to show up before half a year I would just wash, clay, glaze the car and then finally wax/seal. The glaze will be enough to minimize the shown imperfections.
Learning when polish is broken down comes from experience. When you first apply the polish to the paint on a low speed it will haze the paint and make the panel look clouded and white. As you work the polish into the paint it will begin to become translucent and difficult to see. Mezernna polishes can be worked in for a long time. Even though you may feel that you have been polishing for a long time, the polish most likely will not be broken down. You can also tell if the polish is fully broken down by the way it wipes away. Once finished a broken down polish will wipe away with little difficulty. You will begin to understand as you move along the car. It is also helpful to use lights to help guide you in your polishing. Under the light there is a noticeble difference between a worked in polish and one that is already broken down. I would also watch some youtube videos with people using mezerna products to help guide how long the polish should be worked into. Hopefully that will give you an introductory level. Also check out this site for further detailing information.
Dawn soap is only good if you are fully detailing the car. The soap will eat away any previous wax on the paint.
A foam gun is great for letting the soap carry away any grime, dust, or dirt that is on the car. The spray on foam sits on the car and then carry's away dirt allowing you to remove anything on your paint that may lead to scratches when washing it by hand.
If you have any other questions feel free to ask.