My fish just recently got the Ich disease, but I think it's gone now. It was my first time getting that disease but my fish basically cured themselves I think.
So for me, the disease was really easy to get rid of. But another member here, Zoragen, is battling a bad case of it. I think the disease varies with the conditions they're in and maybe even the fish.
Anyway, my 1 betta, 6 neons, and 1 CAE all got it. They're in a 10 gallon tank that stayed heated to 80 degrees, sometimes +1/-1 degree. I heard that Ich doesn't do well in warm temps, so that probably helped a lot I'm guessing.
If you buy the betta, how many gallons is the tank/bowl you're putting him in? Do you have a heater and thermometer for it? And if you really want him make sure you have aquarium salt or some kind of other medicine. I had to used Betta Revive (way expensive for only about 60 drops) for my fish since the pet store didn't have salt.
About the spores surviving. I'm not really sure. I'm guessing that if the disease doesn't show itself on the fish anymore it's probably dead. I was reading around and saw that it's good to continue treatment for at least 10 more days (7 days minimum) even if your fish doesn't have the disease anymore because it could still be floating around in the water trying to build back up.
I'm not all that experienced, but I hope I helped some! :)