First, you really need to both see AND sea trail all sizes of boats. My feeling is that the 16 or 19 are too small to do the proposed trip comfortably. The 22 is a great boat. A lot depends on your physical size. If you are over 6'2" there are real advantages to the 25 from a comfort standpoint (also a bit better ride in the chop). The 25 also has a real head and shower--may or may not be important.
Now as for the route--you need to seriously rethink the Mississippi to New Orleans part of the trip--yes it has been done. But there are virtually no places to get fuel South of Vicksburg MS (and even there it will be jerry jugs from a service station some how. This is distance of 337 miles. The only way to make that would be to run at a slow speed--and take advantage of the current. There are few places if any to safely pull off the river. You will meet a lot of commercial traffic. Even with my background and with a diesel vessle with the range, I would be reluctant to make this run.
A far better plan is to take the route via the Ten Tom Waterway, Tenn. River etc. This is not only safer, but there are multiple anchorages and marinas, plus fuel--although there are some distances which press the range of some of the C Dories.
Let us know what you find as you view these boats. I also would recommend a trip to Wefings, and use Marc's. He has extensive experience not only in boating, but in small pilot house type of boats. This is somthing which may be lacking at some of the other dealers.
I also recommend a visit to:
http://www.nauticalcharts.noaa.gov/nsd/ ... tances.pdf
This is Distances between US ports. There is also the NOAA site which has all of the charts down loadable of coastal areas for free. The Army Corp of engineers also has the electronic charts of the rivers in down loadable or DVD format.