Congrats. I always loved the Flex.
Almost bought an Ecoboost Limited one in 2009 (was 24 hours away from a deal). Until a friend told me to drive the SHO....
The 3.5 has decent power and is a quite reliable engine. I'd also do a trans fluid change every 30k (no matter what they say I think it helps longevity in all autos) and you should get many miles out of her.