For future expansion, I would start right off the bat and get rid of your dc-9's and your B732 immediately. Fuel is getting to the point, that flying these planes will bk you. It looks like you already have too many fleets. I would hate to see your fleet commonality costs
As of going for an all Airbus, or Boeing fleet, you will be waiting a while for your planes. My suggestion would be to slim down your fleets and start ordering B737NG to replace your Airbuses, and MD-80's( I like the MD-80's, but they are real thirsty planes, and maintenance costs are higher). For your CRJ's, and F50's, I would move towards a Saab2000 or the Q400. The CRJ-200's are not really efficient for the amount of pax carried.
As for leasing, if it's a new bird, 2 yrs or less, I usually lease for 10 yrs. Anything older and I lease it until the D-check is due.
That's just my two cents, hope that helps