A number of compatible drives here. Also any quality SSD such as Samsung, WD, or AData. As far as transferring Windows, use a program such as Macrium Reflect Free. It will clone or image the old drive to the new SSD. That program is available here.

If you already did a fresh install of Win10 and it's still fresh, it might be easier to do another fresh install.  Be sure to use GPT partition if you haven''t already.

If you use WD SSD, you can decide which color best fits your needs.  My WD Blue 250GB was a little more cost effective than Crucial 240GB plus came with 5 year warranty.  I've also used Crucial.

SSD is definitely the way to go for a slow running PC.  While 8GB is max listed RAM for the 390, even if you went beyond that, like with another PC, you won't notice a speed increase unless app(s) is RAM heavy.

I recently beefed up a 390 and am wondering if I’m able to run ssd and hdd together as long as I have additional drive caddy. (3.5 to 2.5) Any particular sata cable required? Dell part # for reference?