If they require the credit card - it will block you from web check in. It has done this to me before, but I totally just use the web check in line anyway haha.
You can pre-select seats after purchase, but IMO they're not good unless you're Morning Calm. They often keep only the FAR back of the plane open for regular tickets.
I find that this requirement is disappearing to show cards - if you have Verified By Visa, or using my Korean bank apps with Face ID and certificate - they haven't asked me for any cards the last several times. The one time I did have to show a card, there was advance notice at purchase in red letters and also in the email.
BUT be prepared for it just in case!