You probably only need clothes for her if you are showing. I prefer to keep non-working horses naked and shaggy through the winter. It is really healthier for them. Waterproof sheets/blankets have severe limitations and will only keep the horse dry for a short time in light rain....useful, but don't toss her out in the rain with a sheet and go to work all day. A "dirty" winter coat is actually pretty waterproof. They may look wet, but if you dig your fingers into the hair, it will be dry near the skin, as long as you don't bathe or groom obsessively! (I can be an obsessive groomer, so this was something I had to work on!)
If you are showing, you will be clipping and need to keep that coat from re-growing, so you will need a series of clothing items...a waterproof sheet for turn-out, a cotton sheet for cool summer nights, a couple of fleece liners will make the cotton sheet more versatile....and one big heavy winter coat if you will be showing into fall and she will enter winter without time to develop a very heavy coat.
I worked with show horses in Northern Maine, so I have done both....managed show horses, horses in training through the winter, and horses on year-round turn-out. PA is balmy compared to what we had to deal with, wind-chills sometimes reaching - 80 F!!
I found that different brands fit different breeds and body types better/worse, so you would do well to check with experienced mini owners. I really like Radon, Rambo, Weathabeeta. Don't waste your money on cheap blankets. You will spend a lot more in the long run.