If you'ved decided on DSC, this guide will help you determine what you need.

There are three main boards that are the 'brains' of the system.  PC1616, PC1832, PC1864
For more details please see the attached DSC Alarm Comparison, showing PC1616, PC1832, PC1684

In a nutshell:

PC1616 supports 6 wired zones, expandable to 16 wired/32 wireless, 2 partitions, 4 keypads
PC1832 supports 8 wired zones, expandable to 32 wired/wireless, 4 partitions, 8 keypads
PC1864 supports 8 wired zones, expandable to 64 wired,wireless, 4 partitions, 8 keypads

DSC Keypads are normally wired back to the panel for power/communication.  PK5501eng is the 'fixed message' keypad, and PK5500ENG is the 'full message' version. The PK5500 version also makes programming a bit easier.

If you wish to add wireless sensors and can wire the keypads, you can have *ONE* RFK5501ENG or RFK5500ENG, which are the same with built-in wireless receivers.

If you cannot run wires for keypads, then use a TR5164 transceiver to the alarm panel, and then use WT5500 fully wireless keypad(s) as well as wireless sensors.

Once you determine which main 'system' you need to use, and which keypad(s) are appropriate, it's easy to pick a starter kit with the pieces you need.  Most starter kits will include the main system board in an enclosure, a keypad, battery, phone jack, siren and may or may no include a power adapter. 

From there you can add the power adapter if needed, other wired and wireless sensors as needed. 

Don't forget the wire!  No connecting wire is included for power, keypads or wired sensors, so you'll need some 22/4 also known as QUAD or STATION wire or ZWIRE.
You can find it in 500 foot boxes, 1000 foot boxes, or by the foot.