We thought that replacing them with LED fixtures might be good, but I have no idea what size/wattage would be appropriate, or what a good brand would be. The lights would not need to be any brighter than what we have, and could even be lower. We don't plan on installing a dimmer. I'm not sure if LED lights have any other color than the bluish lights I've seen in refrigerators, etc.

Any suggestions you might have would be appreciated.

You may find the following thread informative

However they sell a thin electrical box to take the romex to a single outlet that is made for placing underneath cabinets, so if you purchased a low voltage kit you could use that box to take your romex to a plug, then plug the transformer into it, and then run the lights off of the transformer. If your cabinets are next to each other you can use one transformer (if sized appropriately) for all lights. If they're spaced apart you'll have to take into consideration that the low voltage cables aren't typically allowed to be fished in the walls. While low voltage, the current may be quite high so check for codes.

At my local big box stores they carry mostly low voltage kits. But search online you'll see more options. Kichler is a name that came recommended to me but comes at a price.