I asked similar questions before purchasing my kettle a little over a year ago and I am glad that I listened to the suggestions to get a 15 gallon vs. 10. Like you, I didn't think I would make any 10 gallon batches, but I eventually did and you can't do that in a 10 gal kettle. As mentioned, buy the biggest, best kettle you can afford, even if it means waiting a couple of months to get the one that you really want vs. the one you can get right now. I hated spending $250 on a giant soup pot, but I will never have to spend that money again.
You can get by without a valve, but I have enjoyed the convenience. It is definitely a nice to have vs. a need to have. I did not get a thermometer, but the kettle has a port for one if I want to add it in the future.