You can use a 1/4" welt foot to sew flat seams, that's what's on my sewing machine 95% of the time. I almost never use a plain presser foot.
Are you sure your model # is Pfaff 1425? The 1425 is a newer expensive machine, and has been discontinued, so parts may be scarce for it. Could it be a 1245?
If so, CLICK HERE
BTW, a 1246 or a 1426 is a double needle machine, not a single needle machine.