For those who know nuts about them, yes, getting a branded one
would save them a lot of headache. Professionals can screw up... a
branded machine. I've screwed up a Dell, HP and Compaq in my
repairs, but DIY PCs repair always go smoothly. 
For those who do know what they are doing, getting a DIY is usually
preferred by them.
Since devices are working, just a few hardware
considerations.
1. Are they going to re-use the RAM? If so, make sure it's
supported by the motherboard. If no, then it's alright. Sell it to
recover some costs.
2. When buying the motherboard, make sure IDE cables are provided
as well, if there are IDE hardware, such as hard disk and CD-ROM
drive. Some motherboards provide IDE support, but they don't
provide the cables. MSI is one of them. In the end, still have to
waste money buying IDE cables.
3. Processor can't be re-used unless the motherboard supports it.
Kindly take a look at the motherboard's manufacturer website for
the list of supported processors for that particular motherboard
model.
4. Graphics card. Gaming users, get a dedicated external graphics
card, rather than rely on the motherboard's integrated graphics
card if any.
5. Form factor. It's obvious what can be done and what cannot be
done.