The Heat would be pretty dumb to go for him unless they get him for cheap or can get rid of Banks' in the process. Personally, I'd look at Chalmers as the starter - he's solid on defense, he's a good distributor, he can shoot and, as shown in the title game, he can hit shots in the clutch. He's also more of a traditional PG, which is something I think they need. With Wade and Beasley around, they don't need a scoring PG - which is what Marbs is, always has been and always will be - they need someone who can get them the ball in good position to score, and I don't know if Marbs is that guy. If I were them, I'd try to land Tinsley before Marbury, or just try and move Chalmers or Banks to the starting role.
I think they expect Marbury to get waived by the Knicks at some point, and would then sign him to the vet minimum. I'm not sure where Marbury's state of mind is these days, but if he buys into being the setup man for Wade & Co. the Heat could make a surprise run in the East.
But that would require Marbury putting his ego aside, and I don't think he could do that unless he's on a team that's going to compete for an NBA title now. The Heat aren't that team. Granted I think they could be a playoff team this year, they're not a championship team, not even the East. The Celtics, Pistons and Cavs are all much better then them, and I think the Hawks could continue to progress and the Sixers should be a hell of a team this year as well with Brand.
Marbury will never put his ego aside. He will think he is the star of any team he would play on. I think signing him would only be detrimental to the Heat.
IMO their best bet is to start Chalmers, and acquire Tinsley to (hopefully without problems) serve as their backup point guard.
If they acquire Tinsley and are serious about winning, he'd be the starter. If he's healthy, which hasn't been often I realize, he's extremely solid and already a proven commodity. He has the ability to give you 13/8, I don't know if Chalmers will be able to put up those type of numbers as a rookie.