$18K should get you a 20 something hp compact with mower and box blade and loader. As you suspect, Kubota and JD are the pricy ones. Mahindra, Kioti, Massey Ferguson, LS would be brands I would look at in addition to the Yanmar. All are excellent brands, they just don't all have the dealer network that JD and Kubota have but each would serve you well.
7 acres is a size that most folks would be looking at 30 something hp tractors but that is just a matter of speed. A 20-28hp tractor using 4-5 foot implements will certainly be able to do the work albeit in a bit more time than a 30 or 40hp tractor using 5 or 6ft implements.
You don't tell us much about what the current state of the land is. Is it overgrown with trees, saplings or brush? Is it simply pasture that needs mowing once or twice year to keep from turning into brush or is it a lawn like pasture that needs mowing every couple of weeks to keep looking perfect??? With a 25-28hp tractor you could run a 5ft bush hog and mow a couple of acres an hour, maybe a bit more. With a 20-24hp tractor you'd want a 4ft bush hog and it would take maybe 2.5hrs to do a couple of acres. Mowing speed will depend on how smooth the land is and how many obstacles you need to dodge.
Bottom line here is that you can get a good tractor for the budget you have but it will likely be under 30hp unless you buy used. Recent used tractors are certainly an option but the problem is that dealers can offer zero percent finance which makes used tractors a bit less desirable. A dealer might have a good trade in and would need to offer a pretty good discount to sell it given that he cannot offer zero finance on the used one. Plenty of folks trade in smaller fairly new tractors when they decide to upgrade so that is certainly something to look for. Used implements on the other hand are often excellent value, especially in the 4-5 foot range you would be looking at. Used equipment in that size is rarely abused (you would have a hard time destroying a box blade or bush hog with a 20-30hp tractor).