If you're building an app, but not using real production data in it a free developer edition org is the best option.
If you're building the app to be distributed to other users (i.e. the appexchange, but you aren't required to use that) you can register as a salesforce ISV partner and get a two-user enterprise edition org for production use. You can create developer edition orgs with enhanced limits from the environment hub feature if you do this. In this case there's a document designed to guide you through this process.
If you are building just for your own use, then you're right that you'd need a force.com admin license. They are, last time I checked, the same price as force.com platform licenses. You must have one admin user to manage the org, so generally one per 50 of your force.com licenses can be promoted to this type at no extra charge. The admin user is actually implemented as a user with full rights to CRM features, and is called a "Salesforce" instead of "Salesforce Platform" license type in your org, but is contractually limited to not use them.
This may vary depending on who you talk to at salesforce, but seems to generally be true.