Until two years ago I was farming in the uplands of Cumbria and, prior to that, moorland in Northumberland. A great deal of the ground on both farms was marshy.
Some areas of this type of ground had been reclaimed in the way that you describe, and indeed some of the ground needed some sort of drainage if it was to be useful at all. It’s a lot of work, environmentally frowned upon these days, and needs ongoing maintenance. And much of the ground does become rushy again, over not very many years, requiring considerable topping and weed-wiping to maintain a balance between grazing ground and rushes. And it’s a balance you want, with the rushes helping to manage the water, provide footing for human and livestock, and also much-needed shelter, especially to lambs and calves.
The right sort of sheep can make use of most ground, I found, but whether you want to choose your sheep to suit the ground, or have a specific type of sheep in mind and wouldn’t want to adapt, could make the difference between managing and not. On the moorland we, and all the farmers around about, had Swaledales and Blackface sheep. I guess in Wales there will be Welsh Mountain sheep, the black and the white varieties. On the upland, still wet for most of the year, and sodden in wet winters, a more commercial Texel-type sheep, bred to the ground with Blackface ancestry, managed - but did take some care.
I would advise driving and walking around the area and seeing what sheep are being farmed there. Talk to some of the farmers about what sheep suit the ground, and how they manage the drainage and the rushes. Ask them what sheep - or other livestock or product - has been on the ground you are looking at buying over the years, and with what success. (We could never understand incoming buyers not asking us locals for our views on the ground before they bought and stocked. Most farmers are helpful folk and would share their knowledge and perspectives willingly.)
But if your heart is set on Zwartbles, or a cuddly Down breed, you might find this ground doesn’t work for you.
Another option might be to use this ground only in summer, or at least not in winter. You can’t get the stocking density on marshy ground that you can on better grassland, but the grass that does grow between the rushes (provided you top about a third each year so they don’t get completely dominant) is sweet, the sheep love it, and the flavour of sheep fattened on it cannot be bettered. If it’s been overstocked through winter, however, the grass will struggle to come through.
On stocking densities; the usual ‘rule of thumb’ for good grassland and average sheep is 5 sheep to the acre. On the moorland we talked in terms of number of acres per sheep! And those Swaledale sheep, which are scrawny and very light on the ground. Both farms had about 2 acres per sheep, but the upland farm also had cattle on the ground through the summer, and a few hardy ones outwintered on some of the ground, including some of the marshy ground.