Force users to fill in specific fields Form data filling

I want to force a user to fill in specific data for a post

For example, the user has to give an image field associated with the post

If they don’t – then the post does not get submitted or saved

I’m not sure where to start with this one, I’m thinking of being brave and trying to write a plugin
But thought there would probably be a plugin out there that does this job already with well written code

ALSO: when the user creates a specific type of post, I want them to have to fill in other fields
For example: price, date and image

Is there a simple plugin that does this?
OK, I know that there’s other plugins that add extra fields that can be filled in – like the All in One SEO pack for example
But the thing is, I would imagine going through all that code would take time!
I assumed there’d be another simpler plugin that did similar to what I wanted

ALSO, where would the extra data be stored?
In the WP_options table?
If the fields are small, they could be stored in tags – but what if the extra fields where HTML or a paragraph of text?

Maybe I could store in main text field and use HTML separators, like <!–more—> for example – and then when displaying, I could iterate through and separate the data??
But this sounds a bit messy and unstable!
I’m sure there’s a better solution?

Any help/pointers for any of the above would be great

Thanks

Omar