"probably 2 desktops and 1 or 2 laptops"

"Word, Excel. Outlook, PhotoShop and Simply Accounting"

Am I correct in assuming that you have no IT department? While building your own PCs is fun, and gives a better value, you'll probably be too busy running your business to deal with fixing issues and dealing with Newegg for replacement parts. Pay the extra money to get the support from Dell/HP/Asus/Lenovo/whichever OEM you choose. Time to solve the issue will be almost identical. You're paying for someone else to deal with it. Budget $500-$750 for each PC and $1000-$1500 for each laptop.

For those apps, you'll get the best experience on Windows. Personally, I'd get Windows 8 so you're running the latest version. Yes, the new start screen takes some getting used to, but if you opt for machines with touchscreens, many of the complaints about clunky mouse interactions disappear.

Photoshop likes memory and CPU, so I'd look at machines with at least 4GB of RAM and an i5 or i7 CPU. The other apps have minimal hardware requirements. Spend more on the hardware now, and they'll last you a lot longer.