1) The DP will act as a primer and to some degree as a sealer. More of a primer when used over metal and more of a sealer used over primer/surfacer. I would consider using a sealer after the second application of DP and prior to basecoat. Sealers are optional and will gaurantee no bleed through and more color consistency. If you want I can suggest one after I look it up.
2) Clear coat is also an option.It really depends on the affect you are trying to achieve. If you are looking for a deep gloss wet look then clear coat it. Apply it wet and don't worry too much about runs, they will wet sand out. I usually apply the last coat of clear with an extra ten percent of reducer for better flowout. If you do not clear, PPG DCC single stage works well and give you an excellent finish. Good Luck!