They're easy to spot, and have a lot of money.
If you follow them around you will find out the whole deal. Taking out who has the gun is first priority.
Crack him in the head with a pipe or something. There are also two spotters and the filler, who can be handled as well.
Three or four half assed actual gangsters could have an easy day of it.
Someday it could be made into a movie with Ben Affleck, Matt Damon, and Brad Pitt playing me (the brains and best looking) of course.