What you refer to as an "antenna" is actually a Global Positioning Satellite (GPS) receiver. What most people recommend on this forum is either the GlobalSat BU-353 or the QStarz Blue Tooth receiver. Both enjoy very good reputations.
The GlobalSat BU-353 can be purchased (on-line) routinely for less than $40. I do not know what the QStar might cost, but surely a bit more.
I, personally, would be more comfortable with the BU-353, because it utilizes a "hard-wire" connection, the unit is rugged, having a weatherproof case and magnet base for external mounting, and it costs less.
Yes, SeaClear II does offer full features, including plotting or "routing." It uses the free NOAA charts. It looks like your boat is sailing on the traditional paper charts.
Have you searched for "SeaClear II" on this forum or the internet?
If you have not taking a boating course, I highly recommend it for your safety and enjoyment. I was a member of the US Power Squadron. Though the name would make you think that the group was only about power boats, they are really more about general boating. They offer all sorts of EXCELLENT training and courses. I am/was a sailor and I learned so much about navigation and piloting. It greatly improved my skills and confidence... that made for safer and more enjoyable sailing.
After you get things set up, you may be able to have your laptop provide a feed to an auto-pilot.
What type of sail boat do you have and what waters do you routinely sail? Are you getting ready to do a big cruise?