I personally think PullUps are the worst thing that could have been invented for kids and parents who are going through the potty training process. I have never once bought or used a PullUp and I not only potty trained my own 3 kids but years ago (just before PullUps came out), I was a preschool teacher and trained dozens of 2-3 year olds. Diapers or underwear- not both or something in between. I'd throw a diaper on my child when they slept until I was sure they could stay dry, but otherwise, once we started potty training, it was underwear and no looking back.
I love the idea of any kind of reminder for you and your child to use the potty often, but I don't see the need to spend money on yet another crazy marketing scheme. Buy a cheap bakery timer instead, or just use the one on your stove if you have one.
By the way, it's unfortunately not up to you (or the idea that she will be starting preschool) to decide when she is ready to be fully potty trained. It's totally up to her. If she is not ready to wear underwear with no PullUp or diaper, she may not be ready to be potty trained. My youngest son was not ready, though he liked to take his diaper off every now and then to pee on the potty like his big brother-- but he did not want underwear yet so I didn't start the potty training. I was a bit concerned with school starting in a few months, but I knew there was nothing I could do. One morning he literally woke up and decided he was ready and has barely had an accident ever since. Not every child is this easy, but I really believe that the ones who are difficult are really just ones who aren't ready when their parents think it's time. Not judging you at all-- just saying she may not be ready and I don't want you to be frustrated.