A Mac would have to be a slightly used model, but I would still go with one over a PC.
Just from my experience owning at least 8 PCs and 8 Macs over the past 12 years.
Most of the PCs have had trouble, some serious, not covered by warranty, while I've had work done on some Mac even after the warranty had expired.
They're also just built tougher.
Neither will run Word unless Word is installed, not sure about Opera.
I'm sure there are Macs for $500, even with a bit of warranty left, and I'd start with eBay (buyers with great feedback and many sales).
http://www.ebay.com/itm/Apple-MacBook-13-3-Laptop-MC240LL-A-AppleCare-/300675769483?pt=Apple_Laptops&hash=item4601ac248b
http://www.ebay.com/itm/Apple-MacBook-13-2-13GHz-2GB-160GB-SD-MC240LL-A-457571-w-Applecare-/120859896655?pt=Apple_Laptops&hash=item1c23cfab4f
http://www.ebay.com/itm/Apple-MacBook-13-3-Laptop-MB403LL-A-2-4Ghz-2GB-Ram-160-Nice-w-Warranty-/170799393289?pt=Apple_Laptops&hash=item27c46fea09
