Have you paddled any kayaks before? If so, do you have any preferences? Will it be for day trips or overnight camping?
I once went through something similar, but for a canoe. I found a local Canoe & Kayak shop where I could get good answers to questions, and I could rent something for a weekend. I quickly learned light weight costs, but it's worth it. Sturdiness also matters, but that depends on the lake where you'll go. I know less about kayaks, but some very light weight canoes are fragile. If the water is rocky, get sturdy plastic.