I upload on average 22 new files every month and at the moment I've sold 296 images (of 354 files online) at least one time. So, it is 83%. But I think this is not so important (and, in my case, with a so small portfolio, not an interesting statistic). I'm trying to upload more and more images.
Then, if I had to choose between 1000up and 500dls and 100up and 400dls, I'd choose the first. This is my opinion.
Your castle is very nice!!