Yup. Good for that, if that's what you need. Doesn't help the hubs to live long and prosper, though.
No, not exactly. But they'd survive street use and mild wheeling for a long time. Anything else and a guy should consider something different anyways. D30 wheel bearings aren't exactly awesome.
They're a cheap option for the junkyard lover.
Hmm, 3.5" you say. 4" has been a little narrow for me.