Rittersteel stuff isn't designed properly as a "real sword" - it's more like they made the blade very thick (and hence poorly balanced)... possibly for countering poor heat treat? I can't say SPECIFICALLY for the Celtic... but the weight alone should tell you something.
You *really* should ask around at SFI, as I couldn't really tell you. I've handled some Rittersteel stuff and they all sucked big time. If you post on swordforum for what you're looking for, time period you're willing to wait, and price range I'm sure people will be able to help you.
I don't know anything about the KC scabbard... but people seem to like it - maybe they know best. I'm planning to get a KC Shingen sometime soon.