custom (simplified) post form

I have one particularly popular and distinct category on my WP blog. I want to create a custom post form for it, so that it’d be easier for users to create posts. Particularly, I want to hide some input fields while add some others (that would just map to “Custom Fields”.

Are there any tips? Any issues I should be aware of? Should I just copy post.php and hack away or there are better alternatives? IIUC I can add new input fields using add_action hooks — but what about hiding some input fields?