Hi Bec

The Paypal thing is very easy to set up - just go to Merchant Services in your account and you can create the payment gateway right there, then install it on your site. Or you could just add a simple 'Pay me with Paypal' graphic that links up to your account - either way, they have great support if you aren't sure what to do once you get into that part of your account. (I actually had to CALL Paypal today for the first time since I started using their services over ten years ago and they were great!)
As to the client login area - that takes a bit of coding and isn't quite as straightforward - although there are lots of client managent systems you can sign up for (external to your site) that will allow you to add client permissions, etc. so they can upload and share files with you.
I have a document library installed on my server and I create custom permissions and folders for any client who needs storage space to send me something or if we have a big project like a website build going on - much easier than trying to grab huge image files etc. via email - after exploring options like Sharepoint, etc. that all have a monthly fee I finally just asked my husband to build me one. It's simple but it works great and I don't have to pay for it. If you're curious about a license and install for your own site send me a PM and I'll set up a guest account for you to check it out.
There are LOTS of great client management systems out there though, check the Resources and Software section of the forums for threads discussing what everyone else is using
If this kind of thing ends up being too much of a hassle, there are always the email services that let you send huge files via email (with a link to down load from the 3rd party site) like Send This File, etc.