What transducer goes with this instrument? And can the instrument give you speed without purchasing

You would need a DST800 NMEA2000 transducer and cabling.  It is sold as a kit here - https://www.boemarine.com/simrad-is42-speed-depth-pack-is42-digital-display-dst800-ducer-n2k-backbone-starter-kit.html

This display will only give you speed if you have a GPS source connected to it, or a speed transducer. But as it comes in the box, and installed by itself, it will not give speed.