I have an E-510 and I have a 4/3 to OM adapter so I can use all my old OM lenses and my old OM T ring, but I would be surprized if you would have any problems getting a 4/3 T ring.
I find the E-510 is a great camera for sport, landscapes, bushwalking and travel. It is light and compact, has live view and 2 modes of IS, etc. I find it is noisy for astro work due to the small sensor size most probably, but if AP isn't high on your list of priorities for the camera it's a wise choice. There are some excellent lenses available for 4/3 cameras if you have the money...
You can do respectable AP with it, but I think you might have to work a bit harder (i.e. collect more data) to achieve comparable results to a larger format Canon or Nikon.
Al.