I suggested the Dimension, as the OP wanted a Flex. I would always choose the Dimension over the Flex. The firmware is hardened.
I'd agree that the G2-20 would do everything, but with the 2.5A PSU, the Dimension will drive the sirens without the hassle of external supplies.
Also for DIY go to Ebay. There are plenty good panels being ripped from defunct businesses or takeovers. The panels are rock solid anyway!