Which strings for Regal R200

[scruff]

I've just bought an old Regal R200 which is basically a Stella/Harmony H929. It's a bit beat up and the action isn't great, but it's not too bad. I'm primarily going to be using it for slide, so the action suits me. I've taken the tuners and the tailpiece off it at the minute in order to clean everything and glue the top where it's slightly lifted from the body. My problem is I'm not sure what gauge strings to put on it. As I said I'm going to be using it for slide, but I'm a bit wary of putting heavy strings on it in case I damage it. I'm not sure what strings were on it when I bought it, but it sounded pretty good.
Has anyone got any experience of this model or anything similar?

Here's a video I found of the exact same guitar.