Which version you are going to deploy AX2012R3 or D365FO , for D365FO you need to provision DEV VM via LCS ,but for AX2012R3 you can deploy Azure VM using Portal It self and install and configure your slef.
My suggestion is to Just Subscribe for the Windows Server VM not For the SQL VM , cost wise its higher.
It's best option if you have your own SQL key (even you can try with Evaluation Key).
For AX 2012 R3 , if it is single user 16 GB with 8 core will be good enough for you to continue with development .
But for D365FO , When you are deploying via LCS , you will have list of compitable VM's , which you can select from the drop down.
Thanks & Regards,
Amith Prasanna
(please mark my answer as verified if this is sorted out your query )