The following advice is only to maximise profit, you may want to keep some of your long haul planes for the sake of a pretty route map.
You should scrap the 707s and 720s to start with. The DC-8s are quite expensive, so I would be considering getting rid of them too. The Super Constellation and DC-7s would be old and are props anyway, so scrap them. I would advise buying some smaller planes, 100 or so seats, such as the 737s on order. 727s are a reasonably expensive, so I wouldn't get too many of them. Basically, you don't want anything much more than 5 years old, or much more than $100 million in purchase price.