If you just buy the land and leave it fallow it is probably a good long term investment..In regards the rice opportunity you can do all the cost analysis you like but if it does not fit into the caretaker system here then its probably invalid and a complete waste of effort..The locals know what that is and will not vary from it.
Usually the caretaker who is responsible for everything and all costs in crop production gets 75% of total profit from rice sales and you the landowner will get 25%.
You should be able to plan for two crops per year but will need to be there at harvest to make sure you get 100% of your 25%!!!!
If you cannot be there then expect reports of pestilence and disaster and requests for new capital for next years seed....
In a good scenario and being slightly consevative your 25% will be approx 25 sacks of milled rice per year..or about 30.000 Pesos.