Although 'plant' in this sense is often 'engins', I have never personally seen that used in this set expression.

Although 'plant' does of course sometimes mean a factory or works (i.e. 'usine' as you suggest), this is not usually soemthing one would refer to as 'hiring' in EN — possibly renting or leasing; but in my view, the use of the verb 'hire' makes it pretty clear we are talking about the other meaning of 'plant'. Of course, your wider context should clear up any lingering doubt...

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Note added at 8 jours (2016-07-12 05:15:42 GMT) Post-grading
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Some companies specializing in hiring only plant of a rather heavier and more professional nature qualify their company title by 'location de matériel BTP', for example, to differentiate themselves from those companies who also deal at the general-public end of the market; just check in Pages Jaunes for all the variants used within this category.