Buying Shards With Crystal's?

If you're going to spend crystals on characters those are good choices. You'll be a step ahead of many on your leaderboard / shard. And you're right, even at low stars they are viable. If I was starting over and spending, I'd splurge a little to at least get those characters to three or four stars as your it'll make your life in arena better and getting those crystals are most important.