Hiya. Just wanted to offer my 2 cents... whatever you buy, remember that all major manufacturer carseats meet the safety standards required by the NTSB. So a cheap one is perfectly safe, if you decide to go that route. It may not be as comfy for you child, the fabric may fray quicker, or it may have fewer "bells and whistles" but I would doubt that it is actually any safer in a car accident than one that costs 3 times as much. I have chosen to go cheaper just because I've done both expensive and cheap, and the extra bells and whistles are not worth the extra $$$. My kids were just as happy in the cheap carseats. (By cheap I mean the ones that cost about $50 at Wal-Mart).
Also, always buy new. Don't buy from a resale shop. You don't know the history or age of that carseat.
I will say, that if you want a carseat that will last a long time for your present child and later your next child, it may be "too old" for the next one. The "life" of a carseat is only supposed to be 6 years (to account for plastic wear and tear and deterioration). Plus, after many years of juice cups, french fries, and spit-up, you may be ready for a new carseat in 2 or 3 years! (Another reason to think about how much money you want to spend).
One last thought; I think if you buy a convertible carseat now for your 9-month-old, that she can use forward facing when she turns 1 and later, then you can use it till she's about 3, then you can get a 5-point harness high-back booster. Heck, you could get that booster anytime after she turns 1 as long as she's heavy enough (in case your next baby needs the convertible seat). Then, she can use it till she's about 5, when she might want something more "grown-up" - my older kids (ages 5 and 7) use the boosters that are just the small square seat things to use with a seatbelt.
Hope that helps.