Location of the stadium is tricky here. The backbone of the fan base will likely be people who grew up playing soccer or families of kids that currently play, as well as hipster types (this is why soccer is so big in Seattle and Portland). Of course there will be plenty of other fans, but I'd bet that >50% of the above types will make up attendance. Soccer families are suburban...hipsters usually in the core or beach. There is also the potential to draw people regionally to games. I grew up playing soccer locally, but my team was in a statewide Premier League, so I have a pretty good idea where soccer is big. Regionally soccer is big at the beach, southern suburbs (Clay and St. Johns), and southern parts of the city. Outside of the metro area places like Flagler County, Ormond Beach, and Gainesville are other hotbeds. There is virtually nothing north of downtown (including the suburbs). Which in this case means that the geographic center of potential fans is somewhere on the Southside. I'd go so far as to guess that >85% of fans will come from south of downtown.
I think an MLS team would be marquee enough to draw people from an hour away to Downtown for games, but no so sure about an NASL team. Of course having the stadium would be a great thing for downtown and I'm a downtown advocate....but I'm not so sure if its the best move for a NASL franchise.