I'm not sure I would associate domain reputation with IP address. IP reputation isn't dependent on the domains being sent. It's based on the quality of the emails among other things.,
A wide open SPF record basically says email might be coming from anywhere, that's not very useful to receivers.
I think what you're really trying to do is find IP addresses with a good Sender Score.
You can make those easily yourself by sending good email.
If bad email is sent the rep of both the domain and the sending IP addresses will be quickly lost, so trying to buy a "good domain" or IP address for sending email won't help much.
I do agree that the domain you mentioned does appear to be sending high quality email from a few different IP address ranges -
Lookup for sunited.com - SenderScore.org