RE: Recommendations: RC electric car/buggy/truck with $400 budget and following criteria
Availability vs Cost. Always an issue. Traxxas is ALWAYS more available locally than anything else. Unfortunately, you pay for it. But that's cool, as long as you're happy with your decision!
The only thing I would recommend is your choice of LiPo. You're new to this, so I'll keep it simple: The LiPo battery you got is rated at a 30C discharge rate. For a Slash 4x4, I'd have gotten a 50C. Put simply, a 50C LiPo is not going to get as warm as a 30C LiPo in a big truck like a Slash 4x4. This is because the higher C-rated pack can deliver more amperage easier, so it doesn't get as hot. In all things electronic, heat is a bad thing. That 50C pack is going to have a longer useful lifetime than the 30C. You'll also notice a little more mid-range punch.
Also, get yourself an IR temp gun to monitor your motor, ESC and battery temps during runs. A motor that is too hot is over geared. What is too hot? On a brushed motor, 140 is too hot. On a brushless motor, 140-150 is normal operating temp, over 160 is getting too hot. Selecting a pinion gear with one or two fewer teeth will lower the motor temp at the expense of top speed. Start poking around the Slash forums for more info.
Oh, and as far as waterproof is concerned. Iwas told that, even tho Traxxas bills their waterproof systems as such, Iam told they aren't, really. So Iwouldn't go will-nilly running your new truck thru puddles. But you should at least be protected well enough that the occasional splash, or a little wet grass, shouldn't cause you any issues. And if you're running on grass, REALLY keep an eye on your motor temps.