Let's look at this: my wife's i7 iMac from 2009 is now starting to exhibit issues with the power supply (that horrid buzzing noise that presages failure before long). It had been replaced once when it was under AppleCare.
It's been out of warranty for quite a while now so I talked to my local computer repair depot today to discuss if it was worth repairing - and they explained that they seldom accept repairs for any computer that old (going on 9 years) as well as the fact that they do not have the power supply in stock to replace it and would need to source it from a third party (if they can find one) and then do the repair. The extra cost would potentially make it untenable. To replace it with a current equivalent is much more expensive than it originally was (I think it originally was originally about $1700 - current models are pushing $2500+ if she wants the i7 cpu). I expanded that machine to 16 gigs of ram too.
I can get her an i5 iMac with current 5k display but with the hardware being from last summer's refresh (or an even older 2015 refurb) the prices are better - she gets a decent 5k display and with a base model fusion drive the price is less overall but I wonder if the performance would be up to par for some of the more demanding software she uses (she does voiceover work professionally and uses a lot of audio software to do edits and cleaning of background noise etc).
It also would be not so great to buy her something new only to have a refresh appear in a month or two.
I'm at a loss at the moment. I feel like we just keep nursemaiding her current machine along until it dies.
Apple's current lineup of iMacs are very nice machines all around, despite what some may say in these forums. That being said, if you can limp along until September/October, I certainly would try. I believe Apple will be releasing updates for both the iMac and the MacBook Pro that contain Intel's latest 8th-Generation CPUs. The core count for these CPUs (Coffee Lake) have been increased, from 2 to 4 for the i3 and from 4 to 6 for the i5 and i7(6cores/12 threads). The GPUs in the current iMacs are not bad at all, unless you are a gamer or do heavy duty video production work.
If your wife's current iMac bites the dust, I would highly recommend either the 21.5" iMac with the 3.4GHz CPU or the 3.6GHz i7 upgrade, upgrade it to the 16GB of RAM (unless she has to have 32GB) and upgrade it to the 256GB or 512GB Flash Storage. The caveat here is that those upgrades (core i7, 16GB RAM and 256GB SSD) put you right at $1999.00 retail.
You might be better off with getting her the 27" i5 with the 3.4GHz CPU, upgrading the RAM yourself (its expensive either way you go) and upgrading it to the 256GB or the 512GB SSD. You end up with the larger screen for not much more money.
Myself, I almost always buy refurbished equipment directly from Apple, save the 15% they take off, and make sure the add AppleCare(+), which on the iMac is reasonably priced. You can find various configurations with Flash Storage or more RAM any given day of the week on Apple's website, but you have to jump on it when you see it as they go fast. Monday and Tuesday mornings or around lunch time EST seem to be the best day/times. YMMV.
The 2017 iMacs are so much faster than the 2009 i7 she has now, I can't imagine that she would have any issues. The Core i7 is probably not even necessary as the 3.4GHz i5 (i5-7500) in the 27" iMac is easily twice as fast as the Core i7(i7-860) in the 2009 iMac. Also, upgrading means she can update to macOS Mojave once her audio software catches up (most vendors are very conservative before they update to support the latest release of macOS).
I focus on the i5 or the i7 in the iMac 21.5" as opposed to the higher-end i5 and i7 in the 27" as the 3.4GHz i5 and 3.6GHz i7 are 65w CPUs and, anecdotally, the fans don't tend to spin up nearly as often as the do on the 3.8GHz i5 or 4.2GHz i7 (95w TDPs) in the higher-end configs of the 27" iMac. Less fans = less noise for voiceover, depending on her studio setup. I'm not sure why you would be worried about performance, if she is currently able to get the job done with a 2009 iMac? Unless the app itself has become a big, bloated mess that eats CPUs and RAM for breakfast. Most voiceover work doesn't demand a ton of plug-ins or tracks the way typical music production does.
Also, flash storage would be faster, size shouldn't be an issue, depending on whether or not she is, or can start, using external storage to keep the audio files and libraries off the macOS System drive. Logic Pro X is said to benefit from this, but I don't know about other applications. I would recommend against the Fusion drive at this point, but if you get one, make sure it's at least the 2TB version, as the 1GB version gives you a paltry 24GB of flash storage versus 128GB for the 2TB and 3TB version.
USB 3.0 storage is fairly plentiful and cheap and should be fast enough, especially some of the USB-C flash drives like the SanDisk 900 or the Samsung T-series SSDs. Best Buy is forever running sales on these drives. Wait until Black Friday, if possible.
If you can do it, I would recommend waiting. If not, you do have solid options. No, they are not the latest and greatest, but they will get the job done for another 9-10 years. Good luck!