You can replace the hard drive with a 2.5" SATA SSD, yes.  But there is no M.2 support, so you cannot add an M.2 SSD in addition to the hard drive.

You may, and you will see a performance increase. You may wish to consider upgrading RAM, since 4 GB is a little light. I would suggest 8 GB, if you can find it at a reasonable price.