Here is an example of what you can get nowadays. Everything is cut at the factory, with some items pre-assembled. This photo shows a 40 ft x 36 ft Great Plains Western model by Sand Creek Post & Beam. Also available with living quarters on the second level.
Would be nice to add open shedrows on both sides so the horses could find shelter from rain or snow without going in the barn. No stalls, no paddocks to keep them separated.
The fence would have to be more substantial if you were adopting mustangs. The rails should be on the side of the posts where the horses are. Pipe panels would be more durable but probably cost more. BLM requires a six foot fence but if you adopt from a rescue or sanctuary that requirement might be waived.