How much can you afford?
For a new airline, particularly one entering a world where much bigger companies are already operating, the best option would be something in the 40 to 70 seats. These planes are usually very cheap, provide decent profits and allow you to enter routes where others are already operating without setting off a price war.
Your passengers don't care how long a trip takes, as long as they get to their destinations. A jet is faster, allows you to create more routes per plane, but is generally a tad expensive. Probably not worth it at the start.
Also, don't waste time and potential profits waiting for the plane of your dreams. Get whatever you can afford and put it to service. This way you'll start growing faster.
If you have enough cash to buy a plane, by all means do it. Fokkers F-27 for little more than €5M or Hawker Siddeley 748 for around €10M are great choices. So are a few others. You should compare specs and prices.
As for leasing - and contrary to many others around here - I believe it is a great choice. It allows you to get planes that otherwise you could not afford. Just make sure you don't go for big jets. Those will hardly (if at all) generate enough revenue to pay for the lease. But anything up to €1M or €2M is usually good enough.
Take care not to take any 1 or 2 months contracts. Those are just traps set up for newbies or players not paying attention.
