FWIW, I bought a Dell XPS8900 from Costco last year.  I added SSD and took the memory from 16 to 32G for not a lot.  It hasn't let me down yet.
 
If I was doing this for a living, Jim would be my supplier for sure.

I’ve always found “off the shelf” PCs to be far too noisy to be in the same room as a microphone, and they generally lack the internal space needed to fit large, efficient heatsinks and to replace the stock case fans with fewer large, slow ones. The video cards in them also tend to have very noisy high-speed fans.

Fan noise is a common complaint on the Dell forum with the new XPS 89XX models.  However, I do part-time PC support and have found the opposite with the PC's I've worked on. I have clients with Dell's (older Inspiron models), HP's and one Acer.  None of these are "noisy" to me (and I'm critical about that).  
 
But, I agree on the other points. Small cases in many models, not enough room in case for much expansion (maybe one hard drive), not enough PCIe slots on motherboards, proprietary CPU coolers and in many cases no room for an aftermarket cooler, etc.  Most are not easily upgrade able (e.g. different motherboard) due to proprietary motherboards, front panel wiring, motherboard I/O shield (on many Dell's its made part of the case and not an insert).
 
 

Generally speaking, with Dell, HP, Acer, etc... you're not going to get Noctua (or similar) CPU coolers, power-supplies with "zero-dB" function, etc.

Maarkr
I've built my own computer since the 486.  Now that my current machine is 4 yrs old, it may be time to upgrade, but DIY prices are HIGH.  I've seen decent i7 machines on the shelf for $600, and a DIY proc, mobo, ram will cost that much.  Granted, I'll have the case, power supply, video card... Saw this article, so, how many are happy with a store-bought machine?  I'd want to use my current drives, so that throws a wrench into it, maybe.
 
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I think it mainly depends on what you plan to do. It can be difficult to classify the home studio hobbyist because you may have one chap who does the rock band thing with drums, bass and he might play guitar himself...so for less than a dozen tracks he's happy and that can be done on a pretty basic setup. Or you might have a chap who like to run dozens of soft synths. Granted you can "freeze" those as you go and save a lot of computer horsepower.
 
But say you're doing huge track counts using unfrozen soft synths so you can keep everything in midi until the end. That means a fast cpu with 16gb memory or more and a second hard drive that's 7200 rpm old school or SSD .That can still be done at a minimal level with run of the mill hardware so long as you don't get too carried away with it.
I bought a 5820K. Overclocked it to 4.4 and it takes everything I throw at it The 5820K probably isn't a favorite compared to some others. 6 cores but shows 12 in CbB with hyperthreading.
 
The main thing to look out for in any new build or store bought consideration is bottlenecks. If the mobo has limited throughput, if the HDD is slow, if the cpu doesn't offer enough lanes, if you can't properly configure your interface because of connectivity issues or software/hardware issues then you'll loose some performance. To use a computer as a basic audio recorder though requires a very minimal setup. You probably won't know you're a power user until you tip the scale. If you're in group 1 you probably need not worry.
 
Most of us want to have enough "just in case". For many here enough is an i5 and a basic setup. I tend to go for a little overkill when I build. I don't often need it, but if I do it's there.

An i3 with a fast clock speed can take a lot, until you load it up.  It just depends on what you plan to do with it.