As you have said that you're looking at buying a business, I would not put as much effort into evaluating the assets worth as I would trying to establish the income that business can provide. What is the net income, cashflow, roi, repayment, etc.
They may be outdated, but if you can get people lined up to use them, does it really matter what they are worth?
Also, for what purpose is this valuation needed? If you are doing this as part of a business plan for a bank, they would be more concerned with liquidation value than whether you're getting a good deal.
That said, if you need to establish a value, it's all about making REASONABLE estimates based on comparable sales. If at all possible, try to find similar, if not identical pieces of equipment. You would then make adjustments based on information available, such as age, condition, features missing/included. If I have more than a handful, I would throw out the outliers. You could also try looking for a dealer in the region and asking them. As you are in Alaska, this process may be very difficult, so again, ask what is reasonable in each case.
As a last resort, I would look at what it would cost to purchase new or comparables in other places and work in the shipping costs.
In the abscence of a thriving market, you're really just making an educated guess, so don't sweat it too much.