How hard it is to replace the rca input on the shindo amplifier?

The $100 Hakko 888D will be just fine for you. It’s, basically, almost just as powerful at the 951. The main difference is the usability and tip options. The 888D will have good tip options if you are doing basic connections and through-hole elements (such as resistors, capacitors, RCA inputs, etc.). You would want the 951 if you need the advanced type tips, such as quad-tip, tunnel, spatula (these are used primary for SMD parts, such as surface mount op amps and ICs). The 951 has a sleep mode where it shuts down power to the iron if you let it sit in the iron holder for a number of minutes (the 888D does not). It also has a different way to set the temperature/settings. The 888D will heat the soldering tip just as well as the 951 and keep it hot.

The following two videos are excellent on the overview/review of proper soldering tip maintenance and using the Hakko 888D:

https://www.youtube.com/watch?v=_exJEnZN9QI

https://www.youtube.com/watch?v=y5weXfQvCgM

It sounded like you were in a hurry to get an iron so that you could solder this weekend, lol.  If you have the patience and budget, I'd say get the 888D.  I would also recommend a 1.6mm chisel tip (part T18-D16 for the 888D).  I have found that it's a really good all-around tip that can be used in a lot of situations.  Much better than the smaller pointed tip that comes with the iron.