B.G.
You are right that keeping your daughter in a five point harness for as long as possible is much safer. I don't know what you budget is, but there are a few seats that will harness her to 65 or even 80 pounds depending on her torso height and how she grows. To find her torso height, have her sit up straight on the floor against a wall. Use a pencil to mark the wall at the top of her shoulder. The measurement from the floor to your mark is her torso height. You can take your measuring tape with you and measure the height of the highest harness slots in different seats. The harness has to be in slots at or above her shoulder (the Radian seats are an exception to this - they allow shoulder to be over the top slots as long as the tips of the ears do not go above the top of the shell). The higher the top slots, the longer she will fit in the seat.
I don't know what your budget is, but there are several seats that will keep her harnessed to 65 or even 80 pounds and last her several years. I have my 14 month old in a Radian80 by Sunshine Kids and I love it. The top slots are 18" and it can harness to 80 pounds. My 9 year old still fits in the harness, but just barely. This seat has an 8 year life span (most seats expire 6 years from date of manufacture) so my 1 year old will use it until she turns 9. If you have a skinny little light-weight child you could even get a Radian65 that has the same 18" slots and can harness to 65 pounds. These seats are narrow and are the only seats with a steel frame instead of an all plastic shell. They do not have a "base" so they sit low in the vehicle and don't look like "baby seats" to an older child. The Radian80 is $250 (we have the Princess cover and love it!) and the Radian65 is $199.
Britax makes the Regent with 20" top slots that harnesses to 80 pounds, but that seat is huge and requires a top tether. It costs $260.
There is an Apex 65 that harnesses to 65 pounds with 17.5" top slots. Then you can take the harness out and use it as a belt positioning booster to 100 pounds. This seat requires a tall vehicle seat back and can usually be found for $130.
Good luck. Feel free to email me at ____@____.com if I can be of any help. I love car seats!