1. It's OK. DVI/HDMI/DP are compatible. HDMI has a lower resolution limit(1920x1200). VGA is analog signal, DO NOT USE IT!
2. Thunderbolt = MiniDP + MiniPCIe The physical interface is the same, so you can connect TB monitor to MiniDP port. But when you do so, you loose other TB benefits(like daisy chaining and USB hub/card reader).
The Dell one is fine if you're OK with 1080P resolution.