I was wondering if the same logic could hold for imac laptop computers?
Theoretically yes. Even Apple themselves sell refurbished models.
But ... computers and laptops being what they are (irrespective of manufacturer), a certain amount of
caveat emptor will be required on your part in order to ensure you don't end up with a lemon. Remember, they are more complex beasts than a phone or tablet.
The amount of due diligence you'll need to do will depend on where you're buying it ... if you're buying it refurbed from Apple, or ex-X from reputable source, then you'll need to do less donkey work then if you're mulling over buying one from a certain well known online auction website.
Do you know what the oldest mac laptop that supports the current operating system?
OS X 10.10 system requirements are openly stated on the Apple website here :
https://support.apple.com/en-us/HT201475
But if I were in your shoes, I would be not be looking at getting my hands on the oldest ... I would try to keep within a 3 year time window if you can afford it (ideally 2 years or less).... basically aim to find the best spec newest one you can afford.
I've got an old Apple laptop siting gathering dust in the drawer that would meet the Apple system requirements ... but I wouldn't want to use it, it would be a little sluggish, at least for my needs, that's despite it having been top of the range when I bought it back in 2009/2010.... we're now 2015, so 2009/2010 is a bit vintage in IT terms.
You have to remember, a lot of developments have occurred in the last few years in terms of processor efficiency and speeds.
Can they support windows office (excel/word/powerpoint)?
Absolutley. You can install VMware and run a Windows virtual machine and run office in that .... I've got Office installed on mine. Microsoft of course also publish a Mac native version of Office.... hasn't quite got all the cutting-edge bells and whistles of the Windows version, but its got more than enough to satisfy most people.