I'm glad you're rethinking rainwater, it's such a cheap and eco-friendly option. As I said before, I use rainwater and my fish breed a lot and are much more colourful and healthier than the same species I've seen in tap tanks.
I have several of them attached to our 2 sheds, and they never get smelly or anything. I have one which is under a rosebush where the water is brown, but that one has lots of live food in it. You'll also get a lot of mosquito larvae in the summer, which the fish will find delicious. These waterbutts are also 5+ years old, and they've not been stagnant or gross at all (probably because rain water is quite pure so there isn't much to go gross) Definitely get the biggest one you can, so that you don't run out too much. I haven't run out of water at all this year, even in summer, when I was just using a small one. I have 2 20L cartons to store the water and bring it inside, which is especially useful when tehre's beena lot of rain and I want to get as much as possible.
When you set the tank up, you could do your first 2 weeks-1 month with full tap, then start doing 50% tap, 50% rainwater so that it's an even split by the time you get your fish, and then you won't waste any rainwater.