6 Discus are better than 4. There is often fighting with fewer than 6.
A 65 will be fine. I hope you're tall with long arms. Otherwise you'll find it hard to clean.
With such a low kH you will either have to do large every day or add a bag of crushed coral to your water storage container.
Good luck. Keep us updated as you go. BTW, I adore that phone box, but I bet that if I had that in my house I'd replace it with another tank. This fish thing is an addiction.