wade is right, but i would suggest going with a spirit over going higher abvI don't know what kind of port I want to make. I just like drinking the stuff and thought a nice fruit port (maybe berries) would be nice. I've only used kits for wine so your advice seems good. I think I may try doing as you suggest.
... i would find a recipe for a berry wine, use about 5-6 lbs fruit/gal and use either vodka (smirnoff works ok for this)/brandy/ bourbon (i use evan williams) to fortify the wine. i think the add spirits impart their good flavors to the fortified wine and the unwanted harshness goes away. you don't need a top shelf spirit to mix, but it doesn't hurt to use something descent. another tip, when you are fortifying it after the wine is finished, don't figure an amount of abv you want and add the spirit accordingly. go by taste! remember though, the wine and spirit will meld and be slightly more mellow with age.
...by figuring percentages of liquids and abv, a good percentage after mixing that we've come up with in our batches (whilst going by flavor), seems to be in the 16-18% abv range. the more fruit flavor, the easier to add more liqour. 6-7/lbs of fruit a gallon would be better suited for the 18-20% range.