Well if you're preparing with an eye to the future and not only the CCNA Security track. You may need to the consider the 5506-x with firepower. For the CCNA Sec, you really just need to be able to configure it for security and understand the basic configuration but the objectives do call out identifying and seeing some of the firepower details.
If you've got the money to do so, by all means get it! but a more cost effective solution you may want to look into would possibly online equipment rental through a lab solution provider. There are pros and cons to this approach. for example, one of the benefits of this approach is that the equipment is setup and some of them even have preconfigured lab guides to help you prepare. The negative of this approach is time limits when you schedule a time, you've got to use it or lose it.
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