SRX Chassis cluster can be formed without license, license is required if you are using any advanced services i.e. UTM, IPS, Appsec, BGP RR, dynamic vpn etc which is valid for stand alone node too, in case of chassis cluster respective licenses must be loaded to both the nodes.
For the failover either use interfaces monitoring (reth interface child) or upstream gateway can also be tracked for the failover using ip-monitoring.
On EX4200 you may required to have two keys for the master and backup role in virtual chassis. also if planning to used more feature set you need enhanced feature licenses (EFLs) or advanced feature licenses (AFLs) for the advanced services.
You can create the virtual-chassis out of 2xEX4200 and can configure aggregate Ethernet (RTG can also be configured if need to avoid STP) for the redundancy purpose.