intro question

Hi,
Just signed up, so am totally new here. I’ve written sites in the past with HTML, CSS and javascript, but am looking here to simplify my tasks a bit. Two questions:
I have a site fairly well-used that offers to do a tax calculation that I have offered for free till now with the request for donations (which don’t come very often).
1)Is it possible here to create something where users need to make a contribution/payment to paypal before having access to the site for 3 months or a year (depending on payment)?
2) Can I keep the pages with the jscript calculations or would I need to rewrite them within WordPress?
Any tips as to whether what I need is possible and where I should start looking would be much appreciated. With a few directions I should be well on my way.
Thx.