Do you have broadband on the first PC? or do you envisage BB in the office?
If so I'd get a BB router a couple of Cat5 cables and n/w cards in the PC's if they haven't got them, and just share out folders via explorer.
If no broadband then buy a network hub, couple of Cat5 cables and n/w cards if needed.
Performance will be no different or not noticeable, and it will only be affected when X pc accesses Y pc and vice versa (obviously the specs of the PC's will affect performance anyway)
You could, but you'd need the same BB software installed on the other machine and swapping the modem back and too also, too much hassle IMO.
Get a router (see my first option) and that will split the BB connection between the pair, if you want to share files you can, and if you want to share say a printer you can also...
do you have a usb broadband modem then, you would need to change to a broadband router and then you networking is solved. printing files should not be a problem speed wise