You can plant right in what you have, you should plant those plants that will do well in your soil conditions. a sunny wet area could be home to a number of nice wetland plants.

Depends on the species you wish to plant. If you want to plant species that require a certain ph or need other conditions controlled it will be more difficult (close to impossible) in that set-up.

If you wish to plant wetland species, blue flag, cardinal flower etc. go for it. You need to find species that will do well in your specific conditions.

I started one like you are thinking...a runoff from my pond. I didn't realize that you could dig it out and add all this stuff to it, I just went online and found bog plants that needed a bit of water on top. It is really nice and the plants, pickerel, horsetail, water nasturium, something called a potato plant, water iris,and mazus are spreading like wildfire. All go down to zone 5-7, so I should be ok in the winter. I found them at eBay or at another place, Az Aquatic Gardens...http://www.azgardens.com/winter_hardy_bog_plants.php
I have had good luck with this company, and they are really good with questions when you call, if they answer! LOL!!

Lesley