why you should have any hassle at all with a broadband conversion. You don't have to buy microfilters for all the phone sockets - only the one which you'll be using for the broadband modem.
Once you've got that up and running you might consider setting up a small wireless network, so the other PC can share the broadband access too. It's very easy to do, and usually works perfectly. In the meantime there's nothing to prevent you from using a normal dial up connection via an analogue modem for PC no.2.
I think you would enjoy broadband,it's a bit like getting a dishwasher - you could function perfectly well without it, but once you've got it you don't ever want to go back.