Hi there-
Don't mix old formula with new formula. Whatever she doesn't finish in her bottle within an hour or so, throw it out.
I second the suggestion to either use a formula pitcher (or another container) to pre-make some formula ahead of time (what you will use in a day or even just enough for a couple of bottles at a time).
Get a bottle warmer. They don't cost much or you can find one used (Craig's list, a second hand sale, etc.) I thought I wouldn't need one, but it turned out to be perfect for warmer up refrigerated or defrosted breast milk, not to mention formula mixed up ahead of time in a formula pitcher.
I would suggest initially offering 8 oz until you figure out how much she consistently takes. My son almost never took a full 8 oz bottle (breast milk, formula, or even whole milk after he turned 1 year!) For a long time we gave him 4 oz., then 5, then 6. Generally, if he finished every last drop we'd offer a little more-but since he didn't like the disruption in his feeding, we would just increase the amount in his bottles when he seemed ready to drink more.
I found the cheapest way to buy formula was with coupons and watching sales. Sam's Club did not carry the formula we ended up using-so we went to Target & Babies R Us the most often. We discovered that different locations of Babies R Us/Toys R Us have different prices for the same formula-so if there's more than one in your area, definitely compare the prices. Sometimes stores offered extra large bonus size cans that, combined with our coupons, were the best deal. We registered online with the formula companies & had a friend who seemed to get TONS of coupons, so she shared those with us. Some folks are happy with the store brand formula. If you feel comfortable with that, then that is probably the least expensive way to go. One word of caution, whichever brand you choose, I would not suggest stocking up until you are certain of which one is going to work best for your baby. My son had latching troubles, so I pumped all of his milk for the first 3 mos. I couldn't keep my supply up, so I began supplementing with formula. At first it seemed fine, but once his diet consisted of half or more formula, I noticed that he started having very very bad gas. He was never fussy, but suddenly he was crying and uncomfortable! He had trouble on Enfamil. We ended up switching to one of the Nestle GoodStart formulas.
Good luck to you!