If you're paying for the stake, then look very carefully at the valuation. Things that make sense to pay for:
- Physical assets (computers, plotters, furniture, real estate, etc.)
- Accounts receivable (projects completed and billed, but not yet paid)
- Projects under contract and in progress
You shouldn't be paying for anything else. You shouldn't pay for Good Will. You shouldn't pay for some portion of the expected earnings over the next few years, etc. Small engineering firms (like the one you're talking about) are based on relationships and name recognition. Those relationships and names are about to walk out the door, so there's little "guarantee" of future work, and most of the future work is going to be won by you and your new partners. You should also keep in mind that you are buying the residual risk from projects completed in the past several years (depending on the statute of limitations for the jurisdiction(s) where the firm has worked).
For the physical assets, pay no more than the depreciated value.
For accounts receivable, keep a few things in mind: the money is owed to the firm, but there's no guarantee you'll get all the checks. So in buying that debt, you're assuming some of the risk. The original owners should get something - enough to cover the cost they put forward in paying wages and overhead for those projects, and some portion of the profit. But as you are buying some of the risk, you should still receive some profit from it.
For the projects under contract and in progress, you should look at it in a similar fashion to accounts receivable. But keep in mind here that the outgoing owners have only covered a portion of the expenses and you'll be picking up the rest. There's also the chance that projects will get cancelled, clients won't pay, etc.
For the last two, you have to figure out what a "fair" allocation of that risk and profit is between you and the outgoing owners.