sync is a great option if you want those stock features and don't plan on changing to an aftermarket headunit. module are available to retain sync, but that's another added cost. personally i would rather get something without it and go aftermarket with those options (bt, usb, . . .) and have better audio performance.
the new focus has a good amount of power and great gas mileage, but is heavy and could use more power to really be fun. however that almost 10k difference between the se and st make a big difference and is deef worth wighing the choices.
the new nav systems are great, but again i would rather have an aftermarket system and save the money on the car payment.
My question for you is this. If you are wanting a bigger car to start a family then does a mustang really fit those needs? the mustang doesn't have much backseat room and will make that difficult with that space and only two doors. I have a 09 coupe and have enough trouble with carseats in that. putting those in doesn't always leave room to put the front seats in a comfortable position, especially for long drives. the mustang also is only 4 passenger, meaning there is not center seat in the back and will require putting the car seat on the side leaving even less room for the front seats to go back.
in regards to the fusion; you can get a 5 speed if you're ok with doing a 4 cyl instead of the v6. they are harder to find, but they are out there. can always add a turbo to get more power if you want it and some suspension upgrades.
2010 Volvo C70
2011 Harley FLHTK
Midwest Crew #22