I strongly advise against using human hair clippers. The reason pet/horse clippers are marketed as such is because they are generally more rugged, better quality than their human hair counterparts, and the blades offer much more length adjustment than their human hair counterparts. The above poster who said to avoid guards on the clippers was also correct. Those aren't really meant for pet/horse applications.
My advice for you since you have a show coming up is to put the clippers away and wait until the end of the show season. Keep an eye out on FB horse product sale groups, ebay, craigslist, etc. At the end of the season is when most people around here get rid of things like clippers. Either they're upgrading to do their winter clipping, or are done clipping until next year when spring hair is coming in and will upgrade then. There are plenty of good clipper (and blade) sales going over the next few months, too, so keep an eye out for those.
For body clipping (anything other than surgical clipping or the more detailed face clipping), I suggest using a 10 blade. MAYBE a 15 blade if there's a great deal and you want a little closer cut around certain areas, like the backs of the ears or around the top of the hoof line. I don't suggest getting a 30 or 40 blade unless you are very experienced with them and have a specific purpose in mind, want something on hand in case you need to clip an area for the vet, or will be doing the more detailed face clipping.
Until you get a new set of pet/horse clippers, I recommend just leaving things as they are or finding someone at the show who has some extra time or will let you borrow their stuff. That's just my own opinion, though.
The wind of heaven is that which blows between a horse's ears. ~Arabian Proverb