There are several daily bus services between Lisbon Sete Rios Bus Station and Batalha operated by the company Rede Expressos.
I have posted copy of the current timetable and prices on ScribD here:
http://www.scribd.com/doc/13626683/lisboabatalha
According to Google Maps, Lisbon Sete Rios Bus Station is 8.4 kms. from Lisbon Airport.
In late afternoon one should allow up to 30 minutes to make the trip from the Airport to Sete Rios Bus Station allowing for traffic.
There are taxis outside Lisbon Airport Arrivals. There may be a bit of a queue depending on the time of day but you should be able to get a cab within 15 minutes.
There are no taxi companies as such. All taxis are either owned by a private individual or a co-operative. There should be no problem getting a receipt. (Ask for a "factura"). However, the taxi drivers may take time to write them out.
Private taxis are usually not licensed to operate outside the greater Lisbon Area. They will take you but if you do not speak Portuguese you are more than likely to be charge a higher price. Prices are difficult to assess. However, according to the online simulator at www.antral.pt (Portuguese Taxi Association), the cost of a taxi for one person from Lisbon Airport to Batalha, (136km according to Google Maps) should be in the order of € 150.00.
A better bet in my opinion would be to book a private transfer.
These are more expensive but probably give you more security.
For example, see
http://www.low-cost-transfers.com/lisbon-airport
