Well, there are a lot of variables here, so no one can really give you a foolproof answer. Firstly:
We need to buy the train tickets in advance
Why? Unless you have some specific reason for this, and given that it's hard to say when you'll arrive at the train station, it seems the safest thing is not to buy them in advance and then get them at Oriente for the next available train. They are unlikely to sell out.
Regarding the airport, it depends. Your plane could be late. If you are coming from the UK/Europe on a budget airline, you will land at T2 and need to take an airport bus to T1, which will take a bit longer than if you land at T1. If you're coming from outside the Schengen zone, you're more likely to land at T1 but then you will have to go through immigration, which could be quick or slow depending on how many other flights from outside the Schengen zone are arriving at the same time.
The airport is pretty small so once you get through immigration, if you have bags to pick up, baggage claim is usually quick (assuming your luggage doesn't get lost), and if you have nothing to declare at customs, you go through quickly without stopping. The metro at the airport is close to the exit from baggage claim and Oriente is not far with no line changes.