1: Most people turn to the SLC's or Germaniums. The SLC comps will cost you somewhere between $250-350. They sound very good for the price. The Germaniums are a step up at around $550. These sound excellent and have an amazing passive crossover design.
2: Most people really like them
3: There are tonality difference between these three models. All have low distortion and excellent accuracy. There are lots of reviews on the Germs, so I will let you search for that. The Profi and Vandium lines differ in that the profi has a paper cone (which gives it the more warm tone that Rainbow is known for) while the Vandium have a metal cones which IMO sound cold.
4: As mentioned above there is a great review on
diymobileaudio.com under the review section. The kick will have exceptional midbass and good midrange. They need to be high passed at 80 Hz on a -12 db/oct slope. The phase plug will have less midbass, slightly better midrange and different off-axis response characteristics.
So, hope that kind of pushes you in the right direction. Honestly, if you are going to spend 500-1000 dollars on a set of components you had better spend some decent time/money on your install. I don't know how much of this you know already...so bear with me.
1: Spend some money on deadening material. I would suggest Raammat for SecondSkin. Get some butyl based mat and some closed cell foam.
2: Might try fiberglassing some kicks. If built right they will give you better imaging. Also, moving from a 2 way system to a 3 way can give you much better results.
3: Try active. This assumes you personally have/know somebody who has exceptional tuning abilities. Otherwise you probably won't get good results.