Someone more knowledge than me can correct me, but I'm pretty sure you could buy a newer bigger drive to replace an older drive. You obviously wouldn't get the full capacity as it would be limited by the other older slower drives. However, you would get a, theoretically, more reliable drive than a random one on eBay. Then as you replace older drives eventually you could have increased size.
Caveat is that it will put stress on the old drive to rebuild, however you'd get that with any drive you put in. General wisdom I saw says to replace all the drives. Although that can be expensive.
I'm any case make sure you have a backup before you do any of the changes.
Not that I'm aware of, but I'm not as knowledgeable as someone else. I'm hoping some quick research or someone better can help. If I remember I can look later on this evening, but can't at the moment.