M8 is still a good radio. Just let go of mine recently.
The Spektrum is a new system that adapts to your M8 or any radio that uses transmitter modules. There's one made for the M8. To make use of it you have to get a Spektrum receiver as well. They sell starter kits that come with both the module and a receiver. The concept is called digital spectrum and works similarly to the way cell phones do. Once you set it up, you won't ever have to worry about someone interfering with your signal again. You can turn on anytime you want at the track and not worry about someone screaming out their frequency being hit. Its also great because you don't have to look for that stupid frequency clip anymore.
I upgraded to the M11, its got some more whistles and bells than the M8 thats all. Your M8 should be good enough. And with a Spektrum, its even better.
Unless you got a lot of excessful money and are looking to have some heated debates at your track on race day, I'd stay away from brushless. Its good for fun but its pretty expensive fun. You can get all kinds of nitro stuff for less than what it would cost you for brushless.