Aloha, good suggestions from everyone. One thing to consider, though is that these grasses may not be temperate-level aggressive, but tropical-level aggressive, meaning can eat you or your house and car, or at least smother your trees in a few days. It is really a different situation if you do not have experience with the crazy tropical grasses, planting trees or even just working with the land can be near impossible with certain species. Perhaps Windy can provide the species so we can get a better idea of what they are dealing with, and location information, too. The really aggressive tropical grasses can form monocultures that can resist grazing, tillage, even moving through them to an unbelievable degree. They are not like temperate grasslands that, if left alone, will become forests relatively quickly, but can maintain themselves as grassland for (in human terms)nearly forever. This was talked about a bit in the early Permaculture texts, probably because in parts of Australia the early designers were dealing with some of the really difficult grasses. This is one reason why temperate-style ranching has been so devastating for tropical forests, because when forest is cleared for pasture, if it is later abandoned, it does not quickly and easily revert back to woodland, but requires active intervention to get out of the grass cycle. Extremely overgrazed areas are actually easier to deal with in some ways, because the aggressive grasses are usually gone by then. There is a difference between knee or waist high tropical grass and the 15ʻ+ tall grass monoculture with 1-2" thick stems, saw-bladed leaves and itchy hairs that we very often deal with in Hawaiʻi. I am only bringing this up to say that it may not be as simple as planting trees and watching them grow, because I deal with the opposite all the time. The grasses we deal with almost always need to be cleared mechanically first, whether with hand tools up to large equipment. Not that I would recommend them, but herbicides are generally not effective unless sprayed from the air, or along the edges over a long period of time, just because no one can walk through them to spray.
One thing I have learned is that if you do any kind of clearing, you must have a plan to occupy all of the cleared space with desirable plants immediately, and maintain your cleared area somehow until they get established, or you will lose your cleared ground and your plants very quickly.
Many of the grasses are rhizomatous, and can propagate themselves from small pieces of stem, so it is not just above ground but below that you need to consider. A big tractor with a flail mower is a great tool to cut lanes in the grass, many people opt for a bull dozer here too, and it is not a bad idea with some grasses. If you can clear wide lanes that you can mow or maintain somehow, you can start a nucleus of desirable vegetation that can cope with the grass, but I would not say that it is something you can plant and walk away from. Any kind of tillage is usually impossible with the grasses I deal with regularly, because they sprout from the stems and are usually too big, tough and woody for anything I know to deal with. After the grass is mulched (2-3 passes with a flail mower) and broken down, some tillage is possible, but that has its own problems in the tropics, usually resulting in lost fertility. By the time the grass breaks down enough to till in, the rest has usually grown back too tall to deal with anyway. I find it works to figure out your guild or companions that will go with each tree and plant everything at once. Perrenial peanut is a good low cover for right around a tree, and pigeon pea and crotalaria species further out. Comfrey grows in the tropics, and it and vetiver can work for a barrier to rhizomes if planted thickly. Plant everything extra thick and cut any extra down later for mulch if it is in the way of your desired tree. If you can fill a 10ʻ circle around your tree with good plants and keep the aggressive grasses out, your trees have a fighting shot at surviving. We also use the kukui tree Aleurites moluccana to take up space when needed. If that is available where you are, it grows fast, provides useful products and lots of mulch, breaks down fast and is easy to cut down and replace with more desirable trees, although it is also desirable in its own right.
Cattle can deal with some of the worst grasses, and goats can eat the stems, but with really tall, thick grass, intensive grazing is difficult. Equipment can make lanes in the grass for access and fencing, to make it easier, but this would not be a quick way to establish a forest. Most people seem to opt for extensive grazing over a very long time, after which the fertility is largely gone anyway and the land will be overgrazed, plus in that kind of management cattle and goats would surely find any trees and eat them too.
I have attached a couple pictures of what I deal with that may give some idea of what grass in the tropics can look like, they are not the most illustrative, but are what I have available right now.
You have your work cut out for you if you opt for a parcel of that size, even a few acres of the really aggressive grasses can be hard work to establish trees in!
I think this lengthy reply may help some that do not have as much experience with tropical landscapes understand how it is different from temperate, and how much more challenging it can be. The title of the post is worth a book or two in and of itself, as well as a lifetime of work for many of us. I think it is worth a lot of discussion, not just for the original poster, but for the entire world at large, based on the impact it could have on the worlds ecosystem.