What you want in a "business class" machine above all else is reliability and longevity. So here are my guidelines.
I stay at least one generation back from the "bleeding edge" on everything. That helps with cost also.
-This is a home user, not a FTSE 100 company!
AMD processor. There's no reason to spend the extra money on an Intel processor. Two cores at 3.0 to 3.5 GHz are usually enough. The last one I built had a motherboard that had a "core unlocker" feature so I got three cores out of a Phenom II but only paid for two. I usually put a third party cooler on the CPU because the ones that come with the boxed CPU's are engineered to save money but usually don't have much head room.
- Just no, this isn't the days of the Phenom and the Core2Duo's. Intel is the best and cheapest choice for an office PC, the motherboards are cheaper and EVERY AMD chip currently sold requires and aftermarket cooler which adds a minimum of £10 to the cost of the chip.
Some OEM's are now producing business class mobo's that are inexpensive. The type of business and what extra hardware peripherals they need may partially determine the mobo. A lot of vendors have stopped including a serial port on the mobo but some older peripherals may need it. Primarily look for reliability and support in the OEM you choose. Staying back 1 or 2 generations here ensures any bugs have been worked out of the BIOS, drivers, etc. This also narrows the choices down somewhat because the vendors discontinued the older products.
-In consumer motherboards they are much more likely to iron out bugs in the BIOS for their current range, not their old range because they no longer care because no one is buying them. Again this might hold true for an actual business but not a home user
I stay away from SSD's for business class machines. They had problems with power outages losing data in the early days. That's been pretty much solved now but the $/Gb just don't work for a business class machine. Don't expect business class users to be conscientious about their backups. Study's show Western Digital and HGST HDD's to be the more reliable. Don't skimp on money here - reliability is the key. Spend a few more bucks to get reliability. I do, however, usually just go with 7200 RPM drives though rather than 10k RPM.
-This is CRAZY TALK, Yes WD / HGST are statistically the better option for a HDD I will agree. (Although they are now both the same company) however again in a home application I wouldn't think twice about using a Seagate or a Toshiba.... well... I might think twice about Seagate. Even so their is no argument for not using an SSD if you can afford it and it suits your storage needs. They are equally as reliable as a HDD in 2015.
RAM capacity and speed is pretty dependent on what the mobo will support and which OS you choose. If they use a lot of older software or are highly dependent on one specific older software application, stick with a 32-bit OS. 64-bit Windows has some hoops to jump through to get older software and batch files to run properly. In terms of RAM, that limits you to 4Gb but if you go with a 64-bit version go to 8Gb because the OS will perform much better.
-This is CRAZY TALK. In 2015 using a 32bit OS is madness, no home use is going to run into a problem in a 64bit environment in 2015
I usually go one or two notches up from what the minimum PSU is for a particular build. That puts less stress on the PSU so it will last longer. If the machine is to be used in a warehouse or other harsh environment I'll go up two notches. Again, don't skimp here because everything else in the box depends on the PSU. You want quality components above all else for the PSU.
-It's a home computer!!!!
Plain Jane cases are good because they save money but it has to have good air flow.
With all of that said, it's really about support. Do you want your friend coming to you for support when things "go bump in the night"? If so, build it. If not point them to a nearby computer shop where they can build a reputation with the vendor. They might want to buy from a "big box" store to save a few bucks but I point out the lack of support going that route. And I definitely tell them to stay away from Dell and HP because they usually have proprietary components that can only be had from them which prevents the user from going to the nearest computer shop for a part to repair a down PC.
- The only thing that really makes sense!!
If you are going to build it you can look at the "Best Of" series here. They usually have business class stuff listed.