Hi,
1. The style packs will indeed have to be redone from scratch as they are not adapted to the new HTML/CSS of HikaShop 3.
If you want the CSS of the buttons you had with your style, then edit the CSS file of your style, copy all the CSS code with the hikashop_cart_button class and paste it in the default style CSS or in your template CSS. You should get back the buttons the way they were.
2. The 'Common Price' text is normally displayed before the retail price in HikaShop.
That's how it was even before HikaShop 3. So if you didn't see it before, you must have had some customization to remove it.
You can use a translation override in order to change that text (and get nothing), or add some CSS to hide it.