our house and some of the land is in a valley, right next to a big river, so obviously we get a lot of water running by heading for the river. My top land is ok, on a slope, but I have two fields in particular which are always boggy, and grow marshy grass. I am fortunate that my OH does land drainage as part of his job, so knows what he is doing ......its just the matter of buying all the pipes etc. which can be costly.
Most of our drains are the old stone ones, and over time they do tend to get blocked, fall in, so water backs up. We have had new plastic drains in one field, and its improved matters a lot. Perhaps its just a matter of drainage with the land you are looking at? Sometimes you can get away with just having the drains repaired, or just put new sections in and join to the old, which can be cost saving.
If land is very wet, I find its impossible to use over the winter, which is very frustrating and means buying in extra winter hay if the animals are in longer because of it.