PS/2 to AT adapters are quite simple, they are just a passtrough so unless the computer uses a proprietary keyboard protocol, it should work

I don't see anything other than VGA that used DB-15 connectors so this is definitely VGA

Mouses are great for point'n click games so this isn't required but it should add some comfort

Warning with the HDD, in the 80's there was something else that used 40 pins connectors. I can't recall the name of it but it was a derived from MFM. Make sure it's really IDE before (even though your 286 sounds like it's a modern one

For now I've never met any fried PSU of that age (I don't have many of them though 🤣). Some of them needed inspections for sure, but all I did was replacing the caps and now these are working perfectly

I think Deksor provided all the necessary feedback.

MFM/RLL drives (probably more correct to refer to them as ST-506/ST-412 interfaces) actually required two cables. A 34-pin control connector (a floppy drive also uses a 34-pin cable) & a 20-pin data connector.
However, some early proprietary CD-ROM drives (like your Panasonic drives) also used a 40-pin data cable (similar to your IDE cable, but they are not the same).

It is possible that your 286 could still be using an MFM or RLL drive, but you can easily spot this if you see two cables being attached to the drive. Otherwise, it's most likely IDE.
SCSI had a 50-pin data cable, but I doubt that you have a SCSI drive in there.

Your 80 GB HDD will not work in the 286, due to BIOS limitations. Either use Dynamic Drive Overlay (DDO) software or, try the XT-IDE BIOS method on a LAN card with a Boot ROM socket.

I would open up the PSU and perform a visual inspection of the caps (although, you cannot always tell if they are bad).
If one of those caps looks like it's bloated, replace all of them.

I recently acquired a 286 from the early 90's and, although the PC still started up, upon a visual inspection of the PSU, it was clear that half of the caps were already bloated and I therefore replaced all of them.
Not very expense, just a bit time consuming, but rather play it safe than sorry.

Yeah that was RLL that I had in mind. Somebody told me that RLL used 40 pins connector, that's weird (though that person has an Amstrad PC so it may just be a proprietary way of plugging the HDD ?)

I would open up the PSU and perform a visual inspection of the caps (although, you cannot always tell if they are bad).
If one of those caps looks like it's bloated, replace all of them.

Yeah, these old caps will not always bulge. Those that were found in my 8088 psu had no bulge at all, but since the PSU was unstable and that the caps came from a cheap manufacturer, I had high suspictions of them being bad and I was right ! The same for the 486 PSU : Unstable PSU, cheap caps with no bulge, caps replaced -> no more instability 😁