Hi,
I would say the basics are one page each for stallions, mares, foals and sales. I don't know what you already have, but you need really good pictures of your horses, but as you said, they shouldn't be to big, because that makes it all load slow. To prevent that, you can e.g. use small pictures (thumbnails) and open a larger version of a picture if the visitor clicks on the thumbnail.
I would add some description to each horse and some pedigree information is always nice. If you write too much though, your visitors probably won't read it.
What I also like is if there is something like a news page to keep you updated on what's happening at your place (e.g. when the first foals are born or new horses are expected to arrive etc). A guestbook might also be nice for visitors to write comments about your website and get some feedback of what you might want to change on a future update
Be sure to make it easy for people to get into contact with you by either adding a contact page with your address, phone number and -very important- working email addy or have that information on bottom of each page.
And please keep away from annoying background music or fancy artwork that will keep the visitors attention from the horses.
Easy navigation is also very important - if the visitor doesn't understand how to get from one page to the next or how to find the information he or she is looking for, than they usually give up fast.
That is all I can think of for now. As you can see, I wrote what I particularly like about good websites and what I don't like about websites that I don't visit often. Others on here may have a different opinion.
I hope that as soon as your homepage is up and running, you'll put a link here for us to see? I love looking at good websites
Carina