I can't speak for the gearing, I don't know much at all about 3speeds. That is a good deal for the two axles though, depending on where you live. Around me Dana 44's are $1000+ and even on Dana 35's people are asking $300-400. I wouldn't care too much about the price of the axles, either way you look at it you are saving hundreds by swapping instead of regearing (If you pay someone to regear). You don't need to upgrade the Dana 30 shafts but new rear shafts could be a good idea. The shafts will be stronger so your spider gears are likely to go first.