2008 Honda Fit Sport, 5MT, 150K miles. daily driver in rush hour downtown metropolis traffic

3 things here

- Getting harder and harder to shift and usually cannot shift into 1st gear or Reverse, have to shut car off, shift into them, then turn back on.. Trusted mechanic without tearing anything apart says it needs new clutch..

- Getting code PO420 on/off, but now is staying on longer and longer,for weeks at a time, then randomly goes off for a few days. New cat or??

- Engine seems to violently shake or at least shudder, usually only coming into and then out of 1st gear.. motor mounts or?


My questions are a follows:

1. Is there a quick overview or guide on clutch replacement? 3 Places quoted me $1900 for the job but I bought the Fit for $2500 so thats a bit rough.. Friend of a friend will do it for $800 if I buy all the parts. Research says this is what i need:
   Clutch Kit + Flywheel
   Clutch Kit + Flywheel
   , but is there anything else I should buy? $800 seem fair?
2. From what I read, clutch replacement is involved process on the 08 Fit since you need to pull the transmission, so anything else I should have address at that time?
3. If I do need a new cat, where to buy from and whats the quick overview replacement process like?
4. No idea on motor mounts or that shakin..
   
   anything else I'm missing or should be aware of?

much appreciated!!