"The effort to relate km or kmt to *handus, or, more generally said, to see a reference to the hands in the number words, is also ad hoc: there is not the slightest evidence, apart from similar speculations on the other numbers, that the Indo-European number words are derived from finger-counting."These comments on poor methodology are as true today as they were then, regardless of whether this old tomfoolery is resurrected on page 316 of Mallory/Adams in The Oxford introduction to Proto-Indo-European and the Proto-Indo-European world (2006 ), albeit subsequently with mild argumentation against the idea.
What then is the current etymology? Apparently no consensus exists yet. For example, The Barnhart dictionary of etymology (1988) says that no cognates of hand exist outside of Germanic. While it's immediately tempting to see an origin in PIE *gʰend- 'to grasp' which yielded Latin praehendere, Greek χανδάνειν and Gothic bi-gitan, formal sound correspondences between PIE and Proto-Germanic forbid us to assume a direct connection with the Germanic root. One would expect a hypothetical PIE u-stem **gʰóndus 'grasper; hand' to end up as **gantuz in Proto-Germanic but certainly not *handuz which rather suggests a non-existent PIE stem **kondʰ-u-. Evidently, these are not the plosives we're looking for and no direct link to Proto-Indo-European appears sensible.
So I had a sudden brainwave and the more I think about it the more sense it makes, although it's frustratingly hard to substantiate. Since there are already a few known Proto-Germanic terms borrowed from Latin in the early first millenium BCE after Grimm's Law had taken place (cf. Ringe, From Proto-Indo-European to Proto-Germanic (2006), p.296), it makes me wonder if one of them might have been our Germanic word in question.
For this crazy idea to stick, we require a Latin word *handus, but as the reader can tell by the asterisk, it doesn't exist (at least as far as I know). On the other hand, prae-hendere 'to seize, to grasp' does indeed exist and the prefix prae- 'before' is secondary. From this implied Latin verb root *hend-, we are certainly free to muse light-heartedly on how we can obtain *handus 'grasper' from it, and very curiously, noting on how it rhymes with the attested Latin word for 'hand', manus. It taunts me with the image of a northern Germanic community with a high degree of Latin bilingualism, inventing new words and idioms out of a faraway language. If only my Germanic-influenced Latin word *handus for proper manus were attested in Roman records, I might develop something more out of this thought.