That is exciting!
There is a volunteer chapter of the SBA, called SCORE (service corps of retired executives) - they have chapters all across the US. I think it would be worth your time to give them a call and see if they can meet with you - I found their experiences very helpful in my own business pursuits.
You will want to evaluate portions of the business differently.
Inventory
Real estate assoc(if any)
Financials and track record (last X years revenue/exp, trends, debts, etc)
Business "goodwill"
You will also want to make your own assessments as to 1) the value of the employees and training they've had and a forecast on how much turnover you will experience as a result of the sale (because you can count on there being *some*)
You will also want to assess the economic conditions and potential future of this business to see if the value will go up or down.
All of those factors should play into the price.
Also, sometimes - depends on the loan, but you can assume a loan in this scenario. If that is the case here, then perhaps you can assume the $150K loan and borrow the rest from the sellers on some kind of owner carry deal...
Best of luck to you.