I wouldn't bother with the mech brakes, I can't tell the difference between motor brakes and mechanical brakes.
As for the smart diff, it should fit fine, though the bearings are going to be a problem - Losi is imperial, and uses funny size bearings anyway. I'd just run a slipperential - the spur is already the right size too, though so is the Losi spur.
I'm running the stock diff oil and it seems to be fine though I am running a stupidly heavy buggy (with a 6S 6100mAh battery!). 4.1kg

. I'll get some pics eventually.
I am working on CAD designs for a quick release battery design similar to that of a cordless drill, it will allow 5 second pit stops, and the use of a battery that is only 356g. It should be good, fingers crossed.