It's interesting to me you just want to do cloth for overnights. Why not during the day too, if that's the direction you're going? Or if she's that potty trained, why not just stay the course w/disposables instead of investing in cloth?
Of those moms I know who do cloth full-time during the day, most of us do disposables during the night, simply because we can't find a good overnight diaper that holds all the urine from 8-plus hours of sleep. My first daughter was a VERY light night wetter, and with her I did a prefold with cover at night (I may have used a doubler, I can't recall) with no problem. She was dry at night very early, way before she started potty training, maybe 2 yrs old? However, with DD #2, she is only 18 months old and has been using disposables overnight for some time, and even those she wets pretty thoroughly.
If you do decide you want to try cloth, great! The other thing is that there are so many types of cloth, it's going to take you a little to figure out what kind is best for your child. My gals have chubby legs so do better with certain kinds; kids with skinny legs do better with others. This may be less of a concern as the kids are older. I know there is at least one online store that offers a trial package of various types, I think for $50, so you can try different types to see what works best for you. A one-size diaper is Bum Genius; they are expensive but work well and are one-size. Personally I don't think they're good for overnight.
Cloth typically only come in newborn, small, medium, large, and don't correspond to number sizes of disposables; cloth generally has snaps or velcro to customize the fit. My first DD maxed out on mediums; I was surprised that she didn't need larges, and the salesperson at Peapods in St. Paul said it goes by hip size, some kids just never need the larger size. (You could check out their website and give them a call if you have questions too, they're really great, alhtough I see you're in IA.)
If you want to try cloth but not at the full cost, you could try buying second-hand on websites like diaperswappers.com or similar. However, I've done it myself, and have found older elastic that has maybe been in storage go after using it two weeks, or people exaggerating the quality of their items. So, try it if you want, but I'd suggest buy new and then you be the one to sell on Craig's List, eBay, diaperswappers.com, etc, later on! :-)
Good luck to you! I really suggest reading all the great websites on this topic out there, and perhaps calling a local shop that sells CD if you have one!
T.
ETA: Oh, and the wool... I did that, but every time the diaper gets poop on it, or whatever, you will have to wash it by hand with special soap, then at times use a lanolinizing agent to re-waterproof it. Again, this works for some people and maybe for you too, but it wasn't worth the work IMHO. I like to just stick it in the wash machine! Wool covers are also more costly; if you know someone who knits, perhaps they could knit some covers for you, which might be cheaper.