I just got my seat back from Sargent, and love it (see photo). The sooner you get it to them, the sooner you'll get it back, but they are super busy right now. I bought a spare seat from eBay so I could ride while they were doing my original seat.
Tank bags are usually either magnetic, strap mount or tank ring mount. A few companies make a tank ring set up, you have to buy the ring, then choose a bag from the same company. You can also get a powered tank ring set up so you can charge a phone, power a GPS in the bag etc, etc. Givi & SW Motech are 2 examples of tank ring bags, they can get rather expensive. Not a bad idea, but I didn't want to have to pay for a ring, and then the bag. You would take the screws out of your gas filler assembly, put the ring over it, and put it back together with longer screws. The bag clicks in, pull a release to unmount the bag. I use magnetic bags. I have a bigger size Joe Rocket, and a Cortech 2.0 for a small bag/every day use. They both stay put, don't move at high speeds, and have not damaged my tank. Bigger magnetic bags will also have a yoke strap just for extra safety that you strap around your steering yoke or bars.
I recently installed a SW Motech center stand. I love it, and use it all the time. Hardest part was getting the springs on. I put a ratcheting strap around my rear wheel, and made a loop out of some thin nylon rope to hook on to the springs (inner & outer). Cranked it until the spring was in position and popped it on the center stand with a screwdriver. Just a tip if you decide to go that route.
I put a small windscreen on my bike, it's made by Puig, but I think you'll want something bigger. I suggest you check out Revzilla or Twisted Throttle for all you want to consider. Amazon & eBay are good source as well, nice to compare prices.
Installed Renthal mediums recently, pretty close to the OEM, just better geometry, and your wrists aren't twisted in so much. Keep in mind that the ID of Renthal bars is smaller than regular ⅞" bars, and most bar ends won't fit. They need to be 12-14mm
Not sure if you know about Sena bluetooth communications for newer helmets, but I installed on in my Bell Qualifier DLX, and love it. They come in handy if you have a passenger, ride with someone, listen to GPS, music, etc, etc
Good luck and welcome!
PS-Check out "what have you done for your bike today" post. You might get more ideas