Never used a Focus one but the AT101W is a great keyboard. I'd get a serial trackball mouse like an old Kensington Expert or a Microsoft PS/2 mouse... or if you want something hassle free there are Logitech M-SBF96 optical PS/2 mice for cheap on ebay. Those work quite well on older machines.

Never heard of the Focus 2001, but the Dell AT101W keys sounds awesome, it is a tactile and clicky keyboard, but it not as loud as the ibm model m.
the DIN to PS/2 adapters will work perfectly fine, both connectors have identical connections, they just differ in physical size and position of the pins, but they are 100% electrically compatible

For mouse, a Microsoft Serial Mouse 2.0a is cheap and readly available to buy(even new) it works perfectly fine, and personally it feels good in my hand, but the mouse that you linked should be nice too, a little bit on the oddware side but should be good i guess.

Can't go wrong with Intellimouse. Low DPI for low-res applications, capable of PS/2 with five buttons, has historical value, and to appease OCD sufferers, it's a Microsoft mouse to be used with a Microsoft operating system.