I think most of the discs will to the job, but I got them once from a construction market and I did not really like them. And once I got one from a friend it was a really nice disc. Sadly enough I can't remember the name anymore. But every disc can do the job so don't worry about that.
Most of the helmets do have an adjustable band, so I would not worry to much about the fitment of the helmet. As far as I can see the helmet you chose will be fine. The quality seems decent.
I would go for a Fluke for sure. I've used them a lot of times and they never let me down. The precision is very good and the device won't get damaged fast. I came to this by using the information on a few different websites. But this page...