It's a start. but the current CCNA Security exam topic list indicates they are using ASA 9.x as the basis for the ASA-related questions:
https://learningcontent.cisco.com/cln_storage/text/cln/marketing/exam-topics/210-260-iins.pdf
You would be better off with an ASA that has 8.4 or later as that marks a significant change in the syntax and operation of NAT and access-lists.
You can also use a VM-based ASAv in unlicensed mode. It is rate limited to 100 Kbps but all the commands and functionality are there otherwise.
All that aside, the hands on bit of ASAs is a very small component of the CCNA Security. Much more emphasis is on general security topics and basics, as indicated in the topic list. Your lab time will be needed more as you advance into the CCNP Security tests.
