Need help starting from scratch.

depends on how long you are willing to wait for parts, and how much you are willing to spend. The amount of colors on your 3MS depends on how many motors you have. You could create a 2 color 3MS if you bought 2 of these: https://www.aliexpress.us/item/3256804737559897.html?spm=a2g0o.productlist.main.9.7f31129deJK8Cf&algo_pvid=88ab8688-cff6-40a7-ae95-d9b95095b70e&algo_exp_id=88ab8688-cff6-40a7-ae95-d9b95095b70e-4&pdp_ext_f=%7B%22order%22%3A%22482%22%2C%22eval%22%3A%221%22%7D&pdp_npi=4%40dis%21USD%213.18%213.07%21%21%213.18%213.07%21%402103205117445050073932886e89f3%2112000034678897920%21sea%21US%216325345170%21X&curPageLogUid=rg3R1AJbDEqK&utparam-url=scene%3Asearch%7Cquery_from%3A

(alternatively, you could also use a BMG clone extruder)

As I said before, it depends on your budget. I am building a 3MS with pancake steppers, a CNC Shield v3, and Creality aluminum extruders, and right now it's only coming out to be ~$60, which is very reasonable for an MMU. ($60 is not including the stuff needed to setup klipper, but the N4+ already has it)

All up to you and your preferences. The only problem you may run into is a filament cutter for the toolhead, and a Bowden fitting that connects to the Extruder, but you could design that yourself.

edit: this looks a bit complex, but you could try adapting this tool head from this ERCF build on an N3+ to the N4+ (then again, I don't know if you are good at 3D design)

Chris Riley has a detailed video (and a 3 video series!) on his channel.

Here is the assembly video:

https://www.youtube.com/watch?v=NUYJseo78tI&t=6s 

This is how I assembled mine.

Side note: print every part on the same plate, so the tolerances are the same with every part.

You have everything needed to make a chameleon unit (as from your list), except for a Y-Adapter. You can print these, but printed ones can catch the filament and cause problems. I would recommend this one: https://www.aliexpress.us/item/3256807550249750.html?spm=a2g0o.productlist.main.1.122dOVH0OVH0zE&algo_pvid=a7093d5b-1e4d-4835-b0b1-03d45d4ae0bb&algo_exp_id=a7093d5b-1e4d-4835-b0b1-03d45d4ae0bb-0&pdp_ext_f=%7B%22order%22%3A%221354%22%2C%22eval%22%3A%221%22%7D&pdp_npi=4%40dis%21USD%216.24%216.24%21%21%2145.46%2145.46%21%402101ea7117444982460931725ec627%2112000042035346114%21sea%21US%216325345170%21X&curPageLogUid=XA9ViyeMYREu&utparam-url=scene%3Asearch%7Cquery_from%3A

(really any Bambu one works)

Since you have a klipper based printer, it may be worth looking at the 3MS by 3dcoded, since it does not need a limit switch to select colors. I am not 100% sure that the USB port on the N4+ can be used to connect an extra MCU, so you may have to do some research. (if you do decide to make a 3MS, don't buy another mainboard and PSU, since you can just flash klipper to a CNC Shield v3 and use that instead).

For the mainboard, I would recommend this CNC Shield kit from aliexpress:

https://www.aliexpress.us/item/2251832035811475.html?spm=a2g0o.detail.pcDetailTopMoreOtherSeller.10.3fe9i0iWi0iW2v&gps-id=pcDetailTopMoreOtherSeller&scm=1007.40196.366991.0&scm_id=1007.40196.366991.0&scm-url=1007.40196.366991.0&pvid=5c83e78e-97fb-48de-a5c7-35db0da5b120&_t=gps-id:pcDetailTopMoreOtherSeller,scm-url:1007.40196.366991.0,pvid:5c83e78e-97fb-48de-a5c7-35db0da5b120,tpp_buckets:668%232846%238111%231996&pdp_ext_f=%7B%22order%22%3A%2210%22%2C%22eval%22%3A%221%22%2C%22sceneId%22%3A%2230050%22%7D&pdp_npi=4%40dis%21USD%2119.99%2119.99%21%21%2119.99%2119.99%21%402103010e17397411430922198e1202%2155305829959%21rec%21US%216242698492%21X&utparam-url=scene%3ApcDetailTopMoreOtherSeller%7Cquery_from%3A

Are you setting it up on a klipper based printer by chance?

Yes. Neptune 4 plus. I’m home now but tired. I’ll post all the stuff I have in the morning if you can help I’ll be so grateful.