I'm not sure that's such a good idea. You'll get $900 if you are lucky for your D300, probably closer to $800 by now, so even if you find a D3100 for $400, which is also unlikely, you only gain $400-$500 dollars towards a new lens. That's only about 25-30% of the cost of each of the lenses you have listed there (well, except, I suppose it covers the 50mm or an 85 f/1.8).
Have you ever used an entry level nikon? You will lose a lot of functionality, and will be diving into the menus a lot for the things that you are used to flipping a switch for. You get one function button for focus mode, metering mode, white balance or ISO (actually I don't even know if you can set all of those with the function button since mine is always set to ISO on my D60). You will have a much weaker autofocus system, and a smaller and darker viewfinder. You also have one button for either AF-L or AF-ON. Finally you drop from 6 to 3 fps and have a weaker battery, plus all sorts of interesting features that you might not use anyway like mirror lock up and an intervalometer. I think it's likely that the IQ improvement you expect would be significantly offset by simply not getting some shots at all. Of course, this does depend a lot on the type of photography you do. Can you tell I really would like to upgrade from my D60?
If you are looking for a small, light secondary body, I would not ditch your primary to get it.
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