If you can live with a maximum screen resolution of 1024 X 768 you can go with the cheapest IMac. (799.00 $)
( I know It's new)
If you need more screen space you have to go for a high end professionnal G4 starting at 1699.00$ without monitor.
You could also go for a used G3 tower model with 17 " monitor that support up to 1600 X 1200 for around 1k.
I advise not to go older than a G3 if you go for a used one. Also do not try a used IMac except if you pay less than 500$.
The best stuff is a beige G3 with at least a 17" screen and crammed with software. Also they come with a decent keyboard and mouse. (new ones stinks)
If you need more info about a used Mac for sale in your local small adds, feel free to ask here about the deal.
The main difference between Mac and Wintel boxes for displaying Web pages is in the font size. Mac displays 2 pts smaller. So I make platform specific CSS.