@Sheena Konas If you're targeting folks who will be bringing horses with them and riding them at the property, you definitely want to make sure your footing and your fencing are top notch. The dressage/hunter/jumper set tend to be a bit more primadonna-ish when it comes to their horses being turned out, the stock horse folks (including the pleasure folks) slightly less so. No one wants their animal to risk injury when out of a stall, but I feel like the dressage/hunter/jumper crowd likes turnout footing more similar to their arena footing whereas the stock horse folks are bit more comfortable with more natural footing in the turnouts. In all cases, barbed wire is never acceptable as horse fencing, the pastures/turnouts/arenas need to be as rock free as humanly possible, and ideally the whole place is cross-fenced so that if/when Houdini makes his escape, he doesn't make it to the road (or highway!).
For arena/work surface footing, you want footing that is as versatile as possible, unless the show grounds cater predominantly to one particular discipline - for example, reiners and jumpers have different footing preferences so you'll want to plan accordingly. Granted, the hard schools will likely happen at the show grounds, but you don't want your clients to feel like they can't work their horses at home because the footing is subpar.
Hot water in the barn area is so, so lovely (my gray (i.e. white) horse wouldn't stand still for a cold water bath which made bathing at shows super annoying for all involved). Ideally feed storage is separate from tack storage.
You want it to be easy for a rig to get in and out. Ideally they can pull in, load/unload, and keep on going without need to back up. It's one thing to turn arond a 2-horse straight load bumper pull. It's another to maneuver a 6+-horse slant, or even a 3-horse LQ (living quarters trailer).
As far as a general show rental, your renters are going to want to be as close to the showgrounds as possible (it's not uncommon to need to be at the barn at 4am to start getting ready if your first class of the day is really early). They'll want to be close to decent restaurants and a large/nice grocery store. They'll eat out quite a bit with their barnmates for dinner, but will likely do breakfast at home and pack coolers to keep at the showgrounds for lunch. Nearby attractions are helpful for non-show days. Competition days are usually Thursday-Sunday each week, but if they're staying for the whole circuit and not competing every weekend, they'll want to sightsee or do fun day trips during their downtime. If your rental is dog-friendly, that's a huge bonus - horse people love to bring their dogs with them!
If the property has hookups, that may be attractive to folks that haul with a camper or LQ. The hunter/jumper folks tend to do either a house or a hotel and don't really do the camping thing as much as the western folks.
Some resources to check out that help provide more context - I highly recommend the Chronicle of the Horse forums (english focused, has forums like BP) and Go HorseShow (western focused, no forums, just articles). The Bay Area Equestrian Network is a west-coast based equine version of Craigslist (super popular resource out here) and has a real estate section with active listings (or you can navigate to the listing agents' sites to see more of their properties).
As @Bruce Woodruff mentioned, the money can border on obscene (basically it's the clients and/or investors that are really the ones paying for all of the fancy stuff the trainers have). Lol. If you're intentional about your offering(s), you can easily charge top dollar!
I hope this info helps. I have a close friend that shows at the big hunter/jumper shows out here (her barn is about to head down to the winter desert circuit) and I work closely with a facility that houses several top trainers that are always coming and going from the big shows. Happy to address any additional questions!