Sorry for not being clear. The slolution moves traffic from the external switch to the SRX so you need to be sure there is capacity for the volume of that traffic.
1. what page should I be looking at on the pdf?
the spec table at the end shows the maximum traffic that can be moved through the SRX IMIX would be the number to check versus all the traffic on the current SRX plus the traffic locally switched on the switch.
2. what exactly do you mean by intra vlan traffic?
I mean traffic between two hosts that are in the same vlan. This traffic today does not hit the SRX. In this new setup it will pass the SRX so needs to be included in your capacity numbers.
3. Overall what is your feeling regarding SRX650 being the only solution?
There are pluses and minuses. I would consider it for a small office.
Plus there is only one device to manage and power.
Minus there will be lower capacity and less switching features as other have mentioned. If you don't need these then it is not an issue at all.
Minus one device failure takes down everything. Also mentioned above cost wise you might get two lower SRX and add redundancy instead of simplicity. Again, your preference on which is more important.
Minus the SRX650 is end of sale so will lose support in a couple years.