While ASA can provide inspection of certain protocols, it does not provide XML inspection. If you configure your traffic policies correctly, you can allow XML communication from outside to DMZ and from DMZ to inside, depending on your requirements.
For your deployemnt, one firewall would be sufficient, but you could use two identical appliances to provide high availability.
Choosing the right firewall depends on other parameters:
- what is the bandwidth required across this firewall
- do you wish to terminate VPNs on this firewall ? if so, how many ?
- how many and which physical interfaces do you require
...
You can find the current datasheets at the links below:
Small and SoHo appliances:
http://www.cisco.com/en/US/prod/collateral/vpndevc/ps6032/ps6094/ps6120/data_sheet_c78-701253.html
Internet Edge appliances:
http://www.cisco.com/en/US/prod/collateral/vpndevc/ps6032/ps6094/ps6120/data_sheet_c78-701808.html