The owner most likely knows what his property is worth as is. I would just ask him if he is interested in selling; what he paid for it doesn't even matter. If it was my house and I paid 8K as you stated, its worth more to me in rent. I certainly wouldn't even consider a lowball offer of 20K. Decide what the place is worth to you and offer it.
As far as the tenants are concerned, if the place is so horrible why don't they leave? I'm not sure about all states but typically as long as you live in the unit you are required to pay rent, regardless of the condition. Typically slum lords aren't pressured by threats. I am not a slum lord and I address repairs immediately; but if someone reported me for repairs I would probably evict them. I would just tell them that I cannot make the repairs right away and give them notice. I would personally take a loss to get them out. Be careful what you wish for.....
Good luck to you and the tenants.