My advice: never buy a used OBi. Most people who own them, keep them as long as they are working. We've seen too many cases of a**holes selling bricked or carrier-locked ones on Craigslist or eBay. You also get a 1-year hardware warranty with a new one. Don't cheap out.
If you want to create the solution in your original post, my answer is...the answer. But, it assumes you already have, or plan to buy either multi-line analog desk phones, or 4 DECT 6.0 cordless systems, one for each user and phone port.
If you have multi-line telephone cable running in the walls, you can hook up both OBi 202s at a central location and use phone jacks around the house. Otherwise, you could use any combination of Ethernet and WiFi (with the optional OBiWiFi dongles).
As an alternate, elegant solution, you could buy as many OBi 1022 or 1032 IP phones as you wish. Each 10x2 can host six different Google Voice accounts, so any family member could make or receive their calls on any of the phones, in any room. Using the OBiWiFi dongles, you don't even need to run wires.