I have been doing a lot of research of late. Oats are not a bad form of grain for horses. Here is a summary of a study done: Research | Equine Feed Oat Project
As well, sweet feed isn't necessarily a bad feed. If it is overly heavy in molasses, yes, it can be high in sugar but some research I have done shows that the actual amount of molasses in a sweet feed is actually less than what the horses can get in hay and pasture. If you go for the textured sweet feed, look at the consistency. All sweet feed will clump together to a certain extent but how hard is it to break apart? The better sweet feed (ducks the rotten tomatoes) will fall apart easily if it even clumps at all where the more molasses heavy sweet feed will remain clumped unless you drop it. Here is an article about the level of "sweetnesss" in sweet feeds: How Sweet is Your Horse Feed? @ Horse Tack Review
Here is another link that is a primer in reading the tag on feed: Publication: Basics of Feeding Horses: Reading the Feed Tag
Pellets essentially contain the same ingredients as a textured sweet feed which means they can also contain some level of molasses. The only difference is that they are crushed and reformed into the pellet so the molasses can't be seen. From what I read, pellets break apart easier and make them easier to digest.
Research has also indicated that a good quality hay and pasture is the best way to go but if you use grain first determine the level of activity you are planning and then use that to do your research.