Camera envy ehh?
First of all take a step back and look at the big picture…you own a D50 and are looking at (maybe) the D200 in the near future while also currently seriously looking at your current lens line up. The D50 is 1.5x crop factor and so has the D200 so as far as lens choices go whatever you go with will have the same effective range on either body. That’s a good point as it means you have some real experience on which you can base your lens choices.
You say that you want fast prime lenses, yet question if the 20 mm will be wide enough. You already have an 18-55mm so you are in the best position to decide if 20 mm is wide enough for you

Also as you rightly pointed out, fast prime lenses are expensive, sure they offer the ultimate in possible quality but depending on what you do with your photos (look at them on the pc, display them on the web, or maybe print them at poster sizes) will be a big factor in how much of that better quality is visible day in and day out. You then need to weigh that against their cost and work out if you can justify the expense.
Macro lenses are a different ball game again but your question indicates that you are mixing up minimum focal distance with maximum zoom. The bigger the zoom the further away you will be able to capture your shot from and (possibly still have a full frame of your subject), however it is entirely possible that a 180mm macro lens may also have a closer focusing distance than some other 60mm macro (this is hypothetical though and I am using it as an example). Basically the minimum focal distance indicates how close you can go to the subject and still achieve focus, while the zoom range gives an indication of the magnification available.
I have a P series Cokin filter system, however I use Lee filters. Filters can and do effect the quality of your photos and if you intend to use this system on prime lenses and then use poor quality filters you will totally negate the benefits of prime lenses and could also introduce the colour casts that OrangePeel recently discussed. Other than the possible colour casts, only you will be able to say if you think the cost vs some loss of quality is as trade off that you are willing to take and if so, that should also make you question your decision to go prime.
I’ll leave your other questions to someone else to pick up on for now
