Sounds like you have the last model, a Verdi Encore.
Absolutely! As good as the Elgar Plus is we have made considerable progress in the intervening 30 years. The Bartók is not only functionally superior but in an entirely different league regarding sound quality.
My suggestion would be to rip them. A Rossini Transport costs about twice your budget and could hardly be justified for 15 discs!
Yes, I believe you can with the Verdi/Puccini/Elgar generation products. It should be clear from the manuals (not all of which I still have). However with the Bartók this would become a non-issue as that would replace both the Puccini and the Elgar (it has a built-in upsampler).
Yes. That would absolutely be my recommendation.
It’s your choice. Roon has lots of benefits over Mosaic which may or may not be important to you, but it is a paid subscription. Mosaic is a little less feature-rich but is included at no additional cost with the product. There are lots of functional differences but no difference in sound quality.
Yes, and that’s entirely doable. For instance the one I assembled for my own system here.