Unless it has already been done, it seems you can expect to replace the axles on just about any vintage unit. So while it should be considered in your calculation of the final cost to restore your trailer, I'm not sure it's really a consideration when comparing the price of one vintage unit to another. Unless of course, as previously stated, the axles have already been replaced, which will save you some cash, but at the same time that cost will probably be added into the selling price by the seller.
Anyway, in my recent research, it seems that the price of vintage units varies widely depending on location. Here in Texas, for whatever reasons, vintage AS units appear to be selling for much higher on average than the guidelines on the "Price vs. Condition" web page at VintageAirstream.com. In other areas of the country, the prices seem to be more in line with those guidelines.
I've looked at several Overlanders over the past month or so, and from what I've seen, and from the pictures you've posted, $3K looks like a pretty good deal. However, you should be sure to check out the subfloor and as much of the trailer frame as possible, because if those are not in good condition, then the price could be too high and you'd have quite a bit of work ahead of you to get it camp-ready.