Actually, they are of the same engine. The B737-500 has two fuel capacity options. Long-range option carries 3726 litres more fuel than standard, thus giving a longer range.
You only need to go to buy, click on the buy button on the list, then there would be a drop box for the engine variants were you could view the specs for each and every option for that aircraft models. This is recommended because as the round goes on, you would see some aircrafts with quite a number of engine / fuel capacity variants. As long as you don't click "proceed" in the order page, you would not be purchasing anything.
