Buying a 6 year old computer of any description is a recipe for failure.
Of what kind? (I'm writing this from a 6 year old computer. I've got even older ones running as servers, an HTPC, and a router...)
The new machine has faster SATA ports, faster CPU, faster memory and a faster GPU in it. it's faster. And will have a warranty.
And it will cost more. If you don't need faster SATA ports, faster CPU, faster memory, and a faster GPU, why spend more money?
Don't get less than 8 GB of RAM, especially with a spinning hard drive.
Bizarre! What does RAM have to do with a
spinning hard drive? I've seen folks here who think that it is just fine to skimp on RAM, and instead spend all day swapping to an SSD, but that's just crazy. An SSD is slower than RAM, and RAM is cheap today; get as much as you need to avoid swapping when running your normal application load. Your SSD will thank you (especially as swapping is the quickest way to reduce SSD lifespan).
4 GB is a sure fire way to kill OS X performance.
Um, no. Actually, even the latest versions of OS X can run just fine in 2 GB of RAM (although, when you do that, it doesn't leave enough memory to actually run any applications without swapping). For light work, 4GB is fine; my mother's machine is running El Capitan with that amount of RAM, and the apps she normally uses (mostly e-mail and web) run fine without swapping.
with a hard drive especially you'll get perhaps 50% of the speed of a machine with 8 GB at 90% of the cost.
Just a flat statement here? Not even going to provide some anecdotal evidence for this claim?
Yeah, it's easy to start swapping in a modern Mac with just 4GB of RAM, but it depends on what you are running.
Replace the hard drive with an SSD as funds permit. It will be a massive boost.
And again, the answer is always to
SPEND MORE CASH!!! An SSD will most definitely be a massive boost -- for any activities that spend all their time accessing the SSD. But stuff like e-mail, web, games -- many apps don't spend all their time reading from and writing to the SSD, and thus do not gain a massive (or, really, any) boost from it...