I am thinking about putting a red dot on my 239 40 S&W. I would like to retain the iron sights so that means having the slide machined to mount the RD. Is there enough metal there to do so?
Any idea if this is doable?

I can't say with the certainty of having done it. But I have now done an RDS on all the other Pseries sigs and I don't see anything on the 239 slide that looks like it wouldn't work. You need someone to do it though, I would ask L&M as my first choice. If you mean retain your current irons there is no chance but suppressor height ones should be fine.