Dang! Sorry about the bad shock, I hope I don't get one any time soon! How many miles are you at?
As far a new suspension, the cheap and quick route would be to just replace that with an oem one. Apparently our rears are much easier to replace than the fronts. I'm not really sure how bad it is to run like that since I've never had that issue. I think Chippy drove around for a while with his blown, or maybe I could be wrong. If it's a major issue driving like that, oem replacement will be your best quick fix (and probably pretty cheap).
If it's not a major issue, save up and just replace everything. Think of it as an excuse to get coilovers HAHA!! You might just want to go with coilovers. I've heard good things about the BCs and they're pretty low price. Also, I'm pretty sure the fronts have adjustable camber (although if the MS3 will never see a track, that might be useless lol). I would love the Mazdaspeed Coils since they are actually KWs, but made specifically for the MS3.
I'm planning on getting coilovers in the future. I think the koni + spring combo probably works well, but for the cost you're very close to the BC coilover price. IMO...might as well just spend the extra money and do that.
Good luck!!!