Morning! We've had Shetlands since 2010 and we milk them, albeit in a prety desultory sortof way. I'd love to recommend them (and I WOULD recommend them, beacuse they are great cattle) but I don't think you have anything like enough land for two horses, 15 sheep and at least two cows. Unless you want to be feeding hay all year round. While you certainly can do and the cows won't mind.
We have 10 acres of grass and two ponies other than the cattle - and we sold all our sheep last year to preserve the grass for the cattle.
While Shetlands are smaller than most commercial breeds, they still eat a lot. Our four in calf girls are going through a big bale of hay every eight days.
If you think of the numbers, two heifers in year one; they calve in year three, so you have two cows and two calves (assume a bull and a heifer); in year four you have two cows, two calves and one stirk, becasue you've sold the heifer calf at weaing; in year five, you have two cows, two calves, one stirk and a beef steer ready to kill at 28 months.
So you don't have two cows, you have up to six cattle beasts.