Welcome to the fun. Please look in the maintenance and repair section at the top 2 threads there. Everything you need to know about your machine is there. If you can't find the answers there, ask questions, this is a great group of SS enthusiasts and woodworkers.
Depending on where you are located used SS jointers can be had for less than $200.
Personally I would put the 520 upgrade on the backburner. Although the
510 to 520 Pro fence upgrade is only $475 from the mothership. Not too bad a deal there.
Except for the PowerPro there is NO motor upgrade available. The 1 1/8 HP model should do you fine.
I vote for keeping the stand alone sander. They are such useful items. I have a 1" strip sander with 5" disc and use it all the time. These are great for sharpening and sanding touch ups.
If you want to process your own lumber also consider the following:
Bandsaw - Less kerf waste and greater depth capacity. Again usually available used for less than $200.
Thickness planer - for accurate thickness dimensions. Look for a good used Dewalt 734 or 735. These are hard to beat in the used market.
Unless you're setup to work outdoors, dust collection should also be considered if pursuing power tool wood working.
Again, welcome to the fun.