Well I definitely agree with the 'low carb' bit, but not *necessarily* with the high fat. Sounds like you're not planning on anything like hard work with her & they don't necessarily need any more fat or feed than a smaller horse(in proportion of bodyweight of course). In fact, being a 'cooler' type, they can be very easy keepers.
So... 'low carb' means low starch/sugar, so grain, molasses, etc should be avoided, minimised or at least fed appropriate for workload. This is for the health of any horse, not just heavies. Even when extra energy is required, there are generally healthier alternatives to grain. Fibrous hay and long stemmy grass is generally a pretty good basic feed, with the addition of a supplement or (grain free) 'ration balancer' or such, to ensure good nutrition. You don't say how young she is, but growing horses tend to need higher protein diets, and lucerne/alfalfa is a good source of this, along with calcium & other nutrients.
If you do find you need to feed her a higher fat ration, sunflower seeds, copra, and even a little straight vegetable oil are 3 options. Any oil will do for weight, but cold pressed, rich in omega 3s are best, such as flaxseed.