automate log in and file download third party website in google script

What we want to do is this: We download excel files (some business reports) manually from a third party website (where we have an account) in our google drive folder. In google spreadsheets we have a script that 1) imports these excel files to google spreadsheets on a regular basis 2) a script that's called from our own site when a user logs in and that sends the queried data. So Google Spreadsheet works as a kind of database for our website. This works great so that part is already solved. Now we want to further automate the downloading part of the files from the third party website. This should also be done in google script so we are platform independant and can easily upgrade code if would be necessary in the future. So the steps needing to be done in the script 1) logging in to third party website. 2) Going to 3 different pages once logged in and set a few parameters. 3)Download the excel files to our google drive account and rename. Preferrably with http request. 4) Import/update to google spreadsheet. This script will be automatically executed daily through setting trigger and is coupled with our google spreadsheet. I already started with the script but my knowledge in this field is too little and don't know how to proceed. Probably a quiet easy task for someone who knows the way, maybe one day of work. Is this something you could help us out with? Please only give reaction if you are certain you could do this in a proper and timely way. Thanx ps. see added word doc. for code where i'm stuck and some screenshots for clarification.