I'm thinking of purchasing a new ASA 5512-x, part number ASA5512-SSD120-K9 and just using it as a plain firewall (as if it was part number ASA5512-K9) and purchase the CX license(s) at a later date when funds are available. Is there any reason I can't do this? I know the SmartNet service contract on the NGFW version will cost about $70 more per year, but other than that will I have any problems?
Sure, that's a supported path. When you're ready you can just order the AVC, WSE and/or IPS subscriptions you want to add. You'll probably want to update the CX boot and system images at that time as they are being updated pretty frequently with bug fixes and enhancements.
The only downside is that you may miss out on some bundle discounts that are typically offered when buying the subscriptions as part of a new purchase.