You picked a good model to start on, it has few parts, a ton of space to add extra details, plenty of detail in the hull to practice detail painting (since you model armor you should be a pro at this), and won't require a lot of rigging and will give you good practice at mounting sails. My first advice would be to go to the fabric department and pick up some good cotton coat thread in tan, brown, and black. This heavy thread is good to begin on because it is heavy and easy to work with. If you want the sails to have a canvas look to them, pick up some coat liner at the fabric department as well, about a 1/2 yard to start with. Use a fabric glue (I use Tacky Glue) and thin it with water and spread the glue on the sail, then lay the liner over the sail and let stand for 24 hrs. ( You can also add battens (these are lines that help the sail hold shape) under the cloth to add extra detail) Cut away the excess and spray paint or airbrush with fabric acrylic whatever color you want the sail. I have also hand painted the cross and other detail on my sail and they look superb. Bear in mind that the sail will be thicker than scale, but I just say that these are the "weather sails" which were thick canvas.
The fit of this kit is very good except you will have a gap where the main deck meets the wall to the foredeck. I just put in the strip of styrene here and it worked out well. The rest of the fit is great, and for a Heller, the masts and spars are very strong. You should enjoy this kit and I recommend it over the Revell Santa Maria.
Scott