Well the best money can buy of course , but since you're starting a new company you probably don't have any. (Trust me, I know)
What type of engineering are you doing? The reason I ask, is if you're not doing very 3D intensive work, you can save some of those precious pennies.
Also, if you are not needing to attract employees anytime soon, you can purchase and older version of Autocad and save hundreds if not thousands of dollars (preferably one that's still supported by Autodesk for the next 6 months). Plus, if you need a "Desktop" suite, count on that doubling the cost of AutoCAD.
Long story short I'm still running the DELL Precision I bought 2.5 years ago. It just fine, but I wish I would have purchased a Gig of Ram and a duel monitor video card. Not that I can't fix that, it's just not that big of an issue.