Seconding Elizabeth K's suggestion to just get decent repros; they'll be more useful long-term, and it's only a wee bit more to save up. What size are you looking to need?
Petticoats: Hem to lower calf, and you're likely to need something in the 140" to 170" or so range to look well. Same with dresses, but lean toward the top end of the scale on those.
Dress Lengths: Because you're still in youthful-length dresses, you'll be able to wear a hoop/cage, or not, without too much trouble. That changes in adulthood.
Hoops: I solemnly swear that I will get the hoops tutorial finished and done and on the main site. It's quite possible to make a simple hoop at home; if you want a ready-made one, there are some nice options out there, and they will reflect the labor that goes into making one. The manufacturer of my favorite hooping style (two thin steels with buckram covering glued on) retired. So that stuff is hard to find anymore. There are other styles still available, mostly from on-line sources. Home Sewing was one source; most places catering to reenactors stock some options, too. They're heavier than the "favorite", but can still work well, particularly in a hoop skirt, versus a cage. They can work in cages, too, but are easiest to handle in cages with casings that contain the hoop, and vertical tapes to arrange and stabilize the rungs.
Truly Victorian has some nice cage instructions for a very reasonable price, and the shaping ends up being very nice. You'll want to leave off the lower rungs (2-3 of them!) to make sure it's short enough for you; the McNaughton sisters are tall, elegant ladies, and not all of us are so tall.