How difficult is it to replace Cat Converter

Ok, so I think my catalytic conver needs replacing. The heat ****eld is
rattling pretty bad, and its pretty old and rusted and I have replaced
or tuned-up just about everything else. So hopefully this will help my
bogging down.  Anyways I have called around to all the local shops
(midas, mieneke, a few local) and the are looking for around $300 to
replace  it. But I can buy the part for around $100 so I am wondering
where the $200 mark up comes from. I know labor and all but I can't
imagine it would take a muffler expert very long to cut off and weld
up a new one. It looks pretty straight forward just remove a few bolts
and clamps and replace them. But I have found many projects to be a
little more complicated than expected.

 So how difficult would it be to do myself, and what surprise parts or
tools should I pick up before attempting.

oh yeah, please don't post "Just cut it off, you don't need it" I know
I can get by with out it. But I want everything in working oder in
case I ever move, have to sell, ect.