"Membership" area on website. Using WP or not?

Hello!

I have made a website not in WordPress. I wanted it custom made so I made it myself.

However I want a “member” area. It needs to be free, it would be for my business and for people working with me.

Here are my questions and what I need. Can someone direct me? I want it the least expensive way. I am not afraid to get my hands dirty as long as there is a comprehensive tutorial to go with it!

1. Can I use WP as my member area while using the website I created without W?. Perhaps putting the WP stuff in a folder off the “main” part of the website?

2. I need to have a sign-up page but I need to be able to customize it. I need more than the regular username, email, address etc fields. I need 3 extra fields specific to my needs.

3. I want to be able to let my members customize their profiles. Bio, image etc.

4. I need to be able to upload files specific to each user. For example invoices and other things.

Can this be done? If not in wordpress then where? I have my own domain and I have a MySQL database set up and I have the My PHP Admin (or is it PHP My Admin) set up with my fields but just couldn’t get past how to link it to my site with the software I am using. I don’t know if the software I am using even can do that so I was looking at word press for this part.

Any suggestions?

Thank you very much!

Rachel