If it transmits to others, it's either an A or B unit. Otherwise, its a Class C (receiver only).

Type A units are much larger, much more expensive, use more power, required dedicaated antenna's, etc. They send-out information more frequently and more of it (rate of turn, desination message, etc.) Type B units give the basic information and it's rate of sending is adequate for a boat going slowly or having the capacity to adjust directions often (tacking, adjusting to waves, etc.).

I would advise you to GET at Type B -- otherwise you will not be "seen" via you AIS. That's often as important for SAR as well as navigation. The cost difference has come way down.

For a Class B unit will need to have a GPS location active. Some have integrated GPS units -- some use your NEMA network to give a location.

You can use a VHF splitter (if the unit isn't integrated into a new VHF) so you won't need a dedicated antenna for the VHF. Also, you masthead antenna will give you greater "range" to receive and send.