Nobody sells you the rainy season, and with land here that is the whole story
I grew up on a family plot outside Chiang Mai, rice on the low part, longan on the high part. My father still farms it. I do soil work for a co-operative now, and I get asked to look at land for people who are buying, usually foreigners, sometimes Thai families coming back from the city.
The question I always ask first is which month they saw it.
Almost everyone views land between December and February. It is the pleasant season. The ground is firm, the light is beautiful, the road in is a road. Come back in September and that same road is the reason your truck stays at the main junction and you walk the last four hundred metres.
What I look for now, and what I tell people to look for:
Where does the water leave. Not where it arrives, where it leaves. Stand at the lowest corner and find the exit. If there is no exit, you own a pond four months a year.
What is uphill of you. Your neighbour clearing his slope changes your drainage completely, and you will find out during the first heavy week.
Old rice terracing. If the land was paddy, it was levelled to hold water on purpose. That is excellent for rice and a real cost if you want a house on it.
Road surface at the last hundred metres. Concrete, laterite or mud is the difference between a year-round property and a seasonal one.
When I help someone compare areas I send them to https://thailand-real.estate/ first, so they can see how differently the same size of plot is priced between provinces before they fall in love with one valley. It is a much calmer conversation when the numbers are in front of you.
And honestly, if you can only visit once, come in the wet season. Everything here is beautiful in January.