I am new to kayaking and would like to buy a used kayak to use at the lake. I have found Necky Kyook, Santa Cruz, and Alsek models as well as a Current Design Kestrel HV 120 in the $800-1000 range.
I know when I go to look at them, I need to check for scratches as well as warps in the hull.
Are there any other factors I need to consider? I will have to lug the kayak across the street to the water's edge each time I go out.
Thanks for your help!

scratches are going to happen but look for abuse scratches they could be long and deep. Make sure that the rudder or skeg operate properly. check for spilts along the seam and cockpit rim. make sure that hatch covers are held securely in place and function properly. check down the center line to make sure that the boat is not bowed to left or right also watch for oil canning in the bottom of the hull. Check the deck rigging for proper fit and function.

Most importantly try it before you by it.

As DGF said, check the keel line for straightness. Turn the boat upside down and sight from stem to stern. Don't worry about the keel being flat; many kayaks have rocker built into the keel. If the keel line is not straight, don't buy the boat.

You mentioned a couple of good brands of boats. I am particulary biased toward Current Design, but there are several other brands that have excellent choices.

Just one quick recommendation: be kind of picky about your first boat. It probably won't be your ultimate boat, but it might have a lot to do with how much further you go with kayaking. I would highly recommend something like an Old Town 138 Loon, or Dirigo. These boats are very well built and can usually be found at exceptionally good prices. I bought my Loon brand new for $450. Even though I now have advanced to sea kayaks, I still enjoy taking the Loon out. The 138 Loon is an extremely capable and thoroughly enjoyable boat. However, they weigh about 60 pounds and unless you are pretty strong, you're probably not going to want to carry one very far.

To second what Magooch said about a boat having a lot to do with how far you go in kayaking. I have sold sporting goods for decades and have often found that people seem to assume that their first mountain bike, roller blades, or whatever, should be small and cheap, as though beginers don't deserve good equipment. You can learn, and advance, in a sport better on good equipment than you can with "entry level" stuff. If you want to paddle on a lake great, but is that all you will ever want to do? Once you prove yourself on the lake you may well hear the call of the sea, or the call of whitewater. This should be a consideration.
Another factor that is often overlooked is who do you want to paddle with? If the people you hope to paddle with have kevlar sea kayaks that are 22" wide and you buy a short, heavy boat 26" wide it will be hard for you to keep up with them and tedious for them to slow down for you. Its not that one type of boat is better than another, just that the two paddling syles are not very compatible. Harleys can't ride with Vespas, although both are fun. Some kayakers are otters, they enjoy estuaries, swamps, protected backwaters and birdwatching. Some kayakers are orcas, they like the long crossings, open water, distant islands and ocean swells. Some of us do a little of both. One should consider buying a boat not just for the paddler that they are but also for the paddler they may become.