If you really want to use your Lap Top as your plotter you may want to look at Captain Voyager. A buddy of mine used it on the Loop and I have it on my laptop just for chart reference but haven't plugged an antenae to my laptop to use it underway. He said it worked great from the Loop to some Bahamas cruising they did.
As far as a fixed mount plotter goes we use the Standard Horizon stuff with C-Map on the Tn. River. The smallest ones are still very usable and can be had for less than $500.
1981 32' Sedan Fly Bridge
440 ci 330hp (2)
7.5 kw Kohler
"International Harbor Marina"
Friendsville TN, TN River