Here is some good information on iron filtration:
http://www.waterfilters.net/Iron-Water-Treatment_c_127.html
It sounds like Solution 1 on that page would be adequate for all of your problems except for turbidity which cannot be removed well by simple filtration.
The DGD-2501-20 filter (which you have) can be used in the first stage in lieu of the DGD-5005-20 (the one recommended in Solution 1).
This should prevent the shower filter from filling up so quickly. I'm not sure which shower filters you are using, but we do have very good options here:
http://www.waterfilters.net/Filters/showerfilters/showerfilters.htm
Are you trying to remove the turbidity in the whole house? If so, it will be fairly expensive. If not, a reverse osmosis system will reduce the turbidity in the drinking water under the sink. Here are the reverse osmosis systems:
http://www.waterfilters.net/Filters/reverse-osmosis/reverse-osmosis.htm.
The third one from the top is the most popular. It is very easy to change because of the "purotwist" technology.
Thank you,
WaterFilters.NET