Elixir 5s are pretty nice, and a huge step up from Vs or the mechanical discs I've used.
I'd say that a lot of the improved performance of a hydraulic system has to do with the fact that it's hydraulic, rather than being subject to friction and housing compression like a mechanical system. So for me, the BR-M445s would have to really suck to make me consider replacing them for an extra $100 worthwhile. Buying a new mountain bike is expensive enough as it is, and you're sure to need to spend money on something, whether it's a stem, saddle, tires, pedals, or something else.
I haven't moved the bike and pressed the brake lever, so I have no idea what echo sound you're talking about. Do you mean there's play within the brake caliper? Are you sure it's not the fork, headset or hub? Play in the brake caliper might be enough suckage for me to consider replacing the brake - I'd be worried about other problems, and I think it would make for somewhat odd braking performance and more noise. But a lot of forks have play, and if the bike hasn't been properly tuned yet, play in a hub or headset isn't that strange a thing to find.
My very first "upgrade" when I got into mountain biking was a set of Avid Speed Dial levers. In retrospect, it was a stupid upgrade, but at least it was in a good place - good brakes really improve the riding experience. You just need to be sure that the OEM brakes don't qualify before you spend money. Shimano's 400-level stuff is a real step below their 500-level stuff, but actually still very nice.