The presser bar, along with the needle bar, should only be changed by a qualified sewing machine mechanic. You will never replace the part yourself without screwing up the timing of the machine, and then you'll have to pay them anyway. $150 for the presser bar and the labor to install it is a reasonable price. The new presser bar is about $30 from a dealer, and it's 1 1/2 hours to replace.
Why do you need to replace the foot? I sew 99% of the things I sew with a welt foot on my machine. Granted, it would be nice to be able to switch feet, but for most sewing it's not necessary.
No one lives forever, the trick is creating something that will.