Welcome aboard. I think it will mostly be an easy transition for you. I think the easiest way to do this would be to just build a custom module- then you stick the module tags on a template and you’re done. How much of it you want to stick in the module really depends on you. The form you don’t even need to make a tag for if you don’t want to- though I generally do. Create an action id to submit to- that will link up to a function in your module to process the submitted form data- and then return the results to the tag.
The developer docs should get you off to a good start. And a lot of it will look familiar- all of your CI libraries/helpers are available (though some are extended to meet EE needs).
The frontend of EE is pretty much it’s own thing as far as display goes. So that will be the biggest change. On the flip side- it’s pretty easy to wrap your head around. But the template engine and use of tags is where you’ll see the different the most. Hence a CI app isn’t totally ‘boltable’. But a lot of it can make the transfer to module form intact (all the processing of the form- and like I say, you can pretty much stick the form straight into a template if you like).
Hope that helps!