iMac on This Roll Top Desk?

Anything goes Al...whatever works for you.

Here's my primary Mac (27" 5K) atop an old nightstand that's only about 20 inches wide and a foot deep on the top shelf. it's only saving grace is being out of the main traffic lane in our living room. Your's will be somewhat safer than mine as it has the rear of your roll top preventing a backwards topple.

Not really, either use a different desk or consider using a wall mounted iMac. If you are in the US, the 21.5" models are at:

Buy iMac with Built-in VESA Mount Adapter - Apple

the 27" models are at:

Buy iMac with Built-in VESA Mount Adapter - Apple

Not to necessarily talk you out of getting an iMac, but you may be better served if you consider getting a Mac mini, and then, the appropriate sized monitor to fit your roll top desk. FWIW, this is what I use for this exact purpose.

I prefer the Saxaphone, as it is closer to the HumanVoice... but we digress. You have two Questions here, actually. The first being the Ergonomic Setup of a new machine and it's display... atop the desk that you are comfortable-with and used-to being in-front-of; when the youngsters get to be our age, only then will they understand 'why' you need to use your existing desk. >>>>> Placing any display-size on that desk top, may have it 'too low' (depending on your existing comfy chair's seat-height/your height, etc). When seated, your eyes should be aligned with your display about 2/3rds of the way up... for most people. The whole point is to avoid neck strain. It can too-easily occur, without you even being aware of it, until a few weeks later when you wake up with locked neck muscles, or have head aches. If you use a slide-out keyboard tray for KB and Mouse, that 'should' place your eyes 'back' away from the display, far enough (depending on your eyesight or use of glasses et al). Again, bigger displays can allow larger text on-screen, but may also cause you to be moving your neck left-and-right, and/or your eyeballs left and right... enough to cause strains and head aches. >>>>> 2ND Question here is reliability of Apple computers. As an Experienced Tech, i recommend you do "extensive" Research before choosing any particular Model; especially an old Mac mini (old Disks, paired with new Sierra or High Sierra macOS, are asking for trouble). [There have been 'rumors' of Apple releasing a new design Mac mini, but nothing solid yet...] And remember that each machine has it's own internal GraphicsAdapter/GPU, which 'should' be paired with a Display that has the same Native Resolution. [***Hmmm... i am having a vision of a large display that lowers from the ceiling on 2 wire cables... and you close your Roll-Top when you wish. Good luck getting the AppleCare youngsters to set that up for you!***]

Alternatively, cosier getting an additional or replacement desk. 🙂

Nice one Al!