ive seen them around this price here and there, i think i paid about that for a pair just over a year ago, so it probly in the right ball park. depending what you want them for the magnisium ones might have been better VFM can often find them for about £750 and they are a lot lighter.
If they are magnesium Marchesinis then yes, they do have a limited life span. The best thing to do to get your piece of mind, is send them to Dymag and get them x-rayed. I know they do Marchis as well as their own. Ring them for costs.
They won't be original Senna wheels, Senna models came with standard three spokers painted red, so if you're assuming they're original kit....they ain't.