First question is what software are you using for your business? If you're using Microsoft Office, an accounting package of some sort, and any other locally installed software you'll need to know what the system requirements are for each software package. The good news is nearly every software package will either have requirements listed on the box or it can easily be found online.
Also think about how many of the applications you use are "Cloud" based meaning they don't run on your computer but are typically websites that you go to in your browser and login. These don't have much if any system requirements on your PC as nothing needs to run on your computer. If your entire suite of business applications is online, then you can save a lot of money by purchasing a computer with lower specs.
As a good baseline here's what I'd recommend for an entry level, mid-range and high performance PC. Note by the time you read this, technology has likely already changed. You'll want to get an i3 processor as an entry level PC, an i5 processor for a mid-range (most common) and an i7 processor if you want serious performance. I'd recommend 8 GB of RAM in any option and unless you plan on storing a huge amount of files locally, a 500 GB hard drive should suffice.
Next take a look at your office layout. What physical size computer would be best? Computers nowadays come in varying sizes from standard tower to ridiculously tiny. Keep in mind however that the smaller sizes are more difficult to find parts for as they are non-standard. This means you can't run to your local PC retailer and buy a power supply in a pinch for example.
So now that you've got the size and performance figured out there's of course your budget. I always recommend buying as high as you can reasonably afford without going into the "Cutting edge" technology that you'll pay a premium for.
Make sure you stay away from big box retail stores that seem to have ridiculously low prices on computers. Although it may seem like you're getting a great deal, here are some things to consider and you'll usually find the big box stores fail at all of them.
- Robustness of the system. Is it cheap and plasticy or built solid?
- How long is the warranty? A commercial grade PC will usually come with 3 years out of the box.
- Does the warranty include on-site support or will you need to drag it back to the retailer?
- Does the warranty or retailer include phone support if you have any questions?
- What version of Windows does it come with? For business you'll want the "Professional" version of whatever operating system is included, ie Windows 8 Professional. This is especially critical if you're planning to scale up your business in future and eventually get a server.
The answers to these questions factor hugely into the price. Remember, the big box stores primary mission is to sell product. Service is not their specialty so don't be surprised if you call to get help and end up quickly realizing the person or the department you purchased from really isn't as willing or able to help as you thought.
Ideally you want to purchase from an IT consulting company. These guys are specialists in technology and work hand in hand with small businesses. They typically provide on-site to your office support, installation and consultation and are more than happy to spec out and custom build you a computer that will match your needs perfectly.
You may pay a bit more but you'll get a lot more bang for your buck and a team of techs at your disposal. They will also typically track your purchases and be able to ensure any future purchases are fully compatible with your technology as your business grows.