RE: Recommendations: RC electric car/buggy/truck with $400 budget and following criteria
traxxas slash 4x4 does meet most criteria.
waterproof
4wd
1/10 (but close to most 1/8)
fairly durable
30mph out of box. faster with a lipo battery
ESC has a 50% mode. can help son get use to truck, then turn it up to full power.
Easy to get parts.
Driveshafts are a weak point. Upgrades availible. Just a bit higher than you price range I think.
Not a ton of RTR's come waterproof.
Axial exo rtr is just over $300, leaving money for batts/charger. Look up video reviews. Its awsome in many ways but does have a few drawbacks.
Redcat caldera 10e. I know you said no redcat, but its 4wd/brushless and pretty durable. Parts are availible all over the web. If you have a local hobby shop, see if they carry redcat stuff. If they do, consider the redcat. If they dont, then just know if you need parts you will have to wait for shipping. Cost of the RTR is $200ish, leaving money for lipos/charger and spares/upgrades.
Ofna lxe kit is a good deal. Just have to do research to pick out the right gear to go into it.
If 2wd is ok, grab a thunder tiger phoenix XT. 2wd brushless buggy for $200. Its a rebranding of the team associated T3 stadium truck. Will do 30mph out of the box...but it comes under geared. Gear it up and 40mph on a 2s lipo is easy. Wheely poping power as well. I have the buggy version. Good sturdy rig for the money. I have 20 lipo packs run threw mine (about 10 hours worth) and only broken 2 parts in that time.