First of all really should be conformation. It is easy to get swayed by a certain color or pattern, or even by a persuasive seller. I have had/heard not-so-honest sellers say things like "well conformation only matters if you are showing". This could not be farther from the truth! Your horse's conformation is what will enable it to DO what you are purchasing it for. Now, if you simply want a yard ornament and a pony for the kids or grandkids to hug on (note, I did not say RIDE), and you are absolutely not going to be breeding the animal, ever, then you can be a bit lax here and let your heartstrings tug you. You might find a rescue that needs help, or just fall for that pretty little face and it doesn't matter that the animal doesn't "look show quality". Those however are about the only situation that is really does not matter. If you are wanting a Mini to use at all-for the small kids to ride, to take to nursing homes, to drive for fun around your neighborhood, etc--then you Really Do need to pay attention to conformation. I will go in depth on this subject in future posts, but for a quick rule of thumb here are a few common mini 'bad areas'.
Hip--if it looks like you can downhill ski off the horse's rump, that is a problem
if the hip is considerably shorter than the back, that is a problem
Back--overly long backs are fairly common, and for a riding mini, are not good
Neckset--if it looks like the horse's neck is coming out of the top of it's legs, rather
than there being a definite chest, that is a problem.
If the neck arches out on the side that connects into the chest, that can be a
problem. This is a common sign of a 'ewe neck', which will cause breathing
issues for the animal. It depends on where and how the neck ties in up on the
horse's back, also. If the whole neck is set forward to far, connecting in too low
the chest and on the back, you have a ewe neck.
Second, if you really want a Mini, Miniatures are never taller than 38" measured at the last hair of the mane. I am seeing more and more newcomers being sold 40-41" grade ponies as "minis". That is dishonest.
Third, temperament. You should be able to safely handle your Mini. Most good minis will come to you, often before you call them. You should be able to halter them, lead them, pick up their feet, run your hands over the entire body. Just because they are small does not mean that they cannot hurt you, I have had my leg fractured by one good kick from a 32.5" mare. That one was my fault, btw. I became lazy in practicing good general horse handling rules, as she was 25 yrs old and a pushover most of the time, and my kids could lead her by the mane and scramble up on her back without a halter or anything and ride her around.
Lastly, general health. It is NOT true that "most" Miniatures founder, have stomach problems, digestive issues, leg problems, stifle problems. Nor is it true that they should not have grain, or alfalfa. You would not buy a full sized horse with these issues, and they should not normally be excused in Miniatures either. I will include "normally" because there are exceptions to every rule-you might be rescuing, for instance. But it is my experience and belief that everything I listed there from founder to stifle are genetic issues. Digestive issues, for example, are almost 100% of the time due to the animal being an actual dwarf, and some of them do not have such pronounced traits as other dwarfs. Dwarfs are NOT Miniature horses, they are a genetic defect, and very costly to keep alive if you are able, and that does not even get into the ethical questions of keeping an animal alive that is suffering every day because some human thinks it is 'cute'.
There are four very general rules for you to keep in mind when you are shopping for your Mini. Hope this helps!