Hi,
1)-After you receive ITA you should upload your relevent documents within 60days, so plenty of time. However you may prepare certified copies/scann-color of your documents such us: birth certificate, passport, references from work, payslips, letter whith positive skill assessment, IELTS TRF, marriage certificate.
2)-3)- when you receive ITA, you have the possibility to creeate an immiaccount directly from skillselect, where you have to fill in a 20pages questionnaire, when you finish the payment is required , which cannot be split (at least I couldn't) and after payment you are able to upload relevant documents. Immi account is not a complicated application.
4) it is recommended not to get your medicals before you are required because they expire after 12month, there are people who get them upfront, it is up to you.
I hope that i haven't missed anything.