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How far have the Iron Hands or Adeptus Mechanicus gone to replace their limbs and organs with bionic ones? Has it ever gotten to the point where their whole body is just a machine with a brain inside of it? Sort of like Robocop (2014).

Yes - that's basically what a Dreadnought is.

One of the rule for techpriests is that they must retain enough humanity to separate them from the Men of Iron, so they probably wouldn't go as far as a brain-in-a-jar plugged into a spaceship, for example. Replacing human parts with mechanical ones is kosher, though, so a humanoid body with an organic brain in it would probably be okay.

It depends and is very inconsistent.
The Fabricator-General of Mars I've seen described as being the most augmented at 70% being machine and in the Lords of Mars Trilogy one Tech-Priest actually IS Brain-in-Jars-level augmented.
It depends on your headcannon since the official fluff is contradicting itself (though I'd lean more on the 70% replacement being the maximum in the current Imperium)

Tiennos wrote:
One of the rule for techpriests is that they must retain enough humanity to separate them from the Men of Iron, so they probably wouldn't go as far as a brain-in-a-jar plugged into a spaceship, for example. Replacing human parts with mechanical ones is kosher, though, so a humanoid body with an organic brain in it would probably be okay.

In many cases, this is what a Servitor is.

I don’t really see how being a brain in a jar would make a tech priest like men of iron to a dangerous extent. Men of iron were explicitly thinking machines, whereas an augmented human no matter the level of augmentation, is still essentially a human even if they are a brain in a jar they have a connection to humanity through shared experience and biology. We have some books like titanicus that I think (if memory serves) have an archmagos and a titan Princepts at over 70 or 80 percent augmented. I don’t think there is an official policy of how augmented you are allowed to be, especially when you have individuals like archmagos of Forge worlds who essentially answer to no one and can tinker on perfecting themselves for centuries.