Hi, I would like to begin using IPv6 just in the LAN with ULA and IPv4 for internet traffic. Can I do that?
I understand Windows will select IPv6 for LAN communications, but
what address will Windows select for internet connections: ULA or Ipv4?
If Windows selects ULA, I will have broken connections and timeouts for internet communications.
Do you think is a good way to begin with IPv6?
IMHO, no, that's not a good way to start. Modern OS's prefer to use IPv6, if available. There are ways to change this, but I don't think you can differentiate between Internet traffic and LAN traffic.
I don't know what kind of router you have or whether you are setting up some kind of tunnel, such as one of Hurricane Electric's tunnel.
If you aren't doing any of those things then you'll have the exact kind of network that everyone in the Windows world has had since Windows Vista was released, as well as *nix and MacOS since some point as well: Dual stack on the LAN but IPv4 traffic on the internet. You'll just be taking it a step beyond that by explicitly configuring IPv6 on the LAN rather than allowing default or automated processes to do it all for you. There is no broken internet connections this way, in large part because there will not be an IPv6 gateway address provided.
Some of my clients I don't do any IPv6 configuration, others (generally those with servers) I explicitly configure IPv6, in part by using ULA addressing. For those with ULA's my intention is to just change the prefixes when we connect them to the internet via IPv6. The key thing is that I've got IPv6 disabled at the firewall/router level, so there is no way for IPv6 traffic to get out to the internet, which forces that traffic to use the IPv6 gateway. This also means that I don't use a Hurricane Electric tunnel (or any other IPv6 tunneling technology) for my clients. I'll only use native IPv6 from their ISP, if and when it's available.