What method would you use to come up with an offer on a portfolio of SFR? Comps, Income, roll dice?
You should have a criteria of what makes a deal work for you. This should be based on Cash on Cash Return, Cap Rate, cash flow, etc. Comps are somewhat irrelevant when buying investments for the long term. Not completely but you should be buying for cash flow with a great return on your money invested. If you are buying to flip these properties then the comps are much more important in that case. You are saving the seller the hassle of selling properties individually though so you should be getting a bigger discount.
My market is very different from yours so I can't speak to your market specifically. I would see what people in your market think and ask advice from experienced investors in your market. Most places though, people can get at least a 10% cap rate / 10% Cash on Cash Return on off market deals. Best of luck!