Miami has a great defense BUT their offense may be in trouble this season so WATCH OUT. If you draft these guys high and the offense keeps doing 3 and out, the defense will wear out and give up alot of points in the second half. A couple of AFC teams have beefed up their run defenses in the off season and it's doubtful that Ricky Williams and the offense will be able to eat up the clock as much (something which makes your defense look even better than it is). This is a powerhouse team but it has virtually no depth at offense. A single injury on offense could turn a 10-6 season into a 6-10 season. I'm not saying stay away from Miami (I'd be an idiot) but I am saying don't mortgage the farm to get them. If you get Miami, who will probably go strong for at least half the season, try to also draft a team which should blossom during the second half of the season. Then if Miami falters, hopefully you can plug in your sleeper replacement.
Using free agent Defenses each week CAN be done but let's not pretend that it's not alot of hard work. You have to know exactly how each defense stacks up against each offense. You have to know the players well enough to know why TEAM A, with the best receiver corps in the league, can't throw against Team B, despite that fact that mediocre passing TEAM C just threw for 400 yards against TEAM B. You have to understand coaching strengths and weaknesses and how "up" players are likely to be for a particular game. Otherwise, you're snatching at defensive yo-yo's and you may catch them at the bottom of their game more often than at the top of the game (if they were consistently great on defense, they wouldn't be in the free agent pool).
The first thing you have to do when researching who to draft on defense is to close your mind to all the hype. Especially during preseason. Look at last year's defenses, look at this year's schedules, and compare how changes on offense and defense may affect each matchup. This would take forever to do for each team, but you all ready know four or five teams you'd take in a heartbeat, so you basically just have to analyze another handful to see who might be worth taking if you miss one of your targets.
Defensive Sleepers - They're sleepers because noone knows what they're going to accomplish. You want to gamble on one but you don't want to gamble very high. Basically look for a D that has all the ingredients but needs time to gel.
Defensive Free Agents - Try to get a feel each week for how each team does, both offensively and defensively. If a low ranked defense loses by a small score like 14 - 10 to an offense like San Fran, that's a good indication that the defense is worth grabbing. If you don't pay attention to what the Free Agents are doing each week, they may become someone else's superstars. This is true at all positions. If you see a coach losing faith in his offense (perhaps by punting on 4th and 1 from the 50), that may be a good sign to grab that team's kicker.