Purchase Requisition within Sharepoint: Form or List?

We have been using a fairly limited document library system within Sharepoint 3.0 to raise, submit, approve and archive Purchase Requisitions within out company.

I created a near exact copy of our old carbon copy reqs within Excel and gave it some functions to calculate totals and budget holders etc. It works, but it's still a little clunky and not 100% foolproof.

I'd like to look into upgrading this to a more stable method and I was wondering if there would be any experts out there who could point me in the right direction - translate the document into a form with InfoPath, something I don't have a lot of experience with, and I believe each user would need Infopath Fillrer to edit it? Or into a List which i believe would be a bit of a fiddle to get working, but does do away with a document.

I have included a copy of our purchase requisition below, please cast your expert eyes upon it :)
purchase-req.jpg