What camera do you use for interiors?

[Johann Jells]

I want to up my game. I have a photography and art background, so I'm not looking to hire anyone, but I want to do better than compact cams or a phone with a wide attachment lens. I know I can get an DSLR with a wide prime lens, but it seems a silly to spend $1000 or more when I would rarely use it for anything other than CL ads for my rentals. 

So, is there any camera people are using other than an interchangeable lens DSLR or viewfinderless with a wide prime? What would be considered an optimal focal length? (obviously 35mm equivalent)