You won’t have to get to a band D before installing solar, but you will need to be a Band D when you submit your FIT application. A 4 KWP system adds between 9 and 17 points to your assessment depending on the aspect.
If I were you I’d want to see a draft EPC before committing…..and a prospective EPC With the solar added just do you know. They don’t need to lodge the prospective one….but just produce a draft one so you can be sure.
It’s what I do for all my prospective pv customers and there is no charge. It’s all part of the service.