We will begin with the bottom line: You should not make a beracha. You have already identified some of the several doubts about the appropriateness of a beracha in this case. One does not make a beracha unless there is a strong likelihood that it is called for (safek berachot l’hakel), and that is not the case here. We will now take a brief look at some of the indications on various doubts.
There is a machloket Rishonim regarding whether one ever makes a beracha on the mitzva of ma’akeh. The arguments against making a beracha include the following: The mitzva is performed to remove danger, rather than as a classic positive mitzva; it is mitzva that is rooted in natural logic;[1] and there is a concern that the one building the ma’akeh will not carry through and actually complete it. Nevertheless, despite the principle of safek berachot l’hakel, there is enough of a consensus of Rishonim and Acharonim to justify reciting a beracha on this mitzva in general.[2]
Your case differs, however. First, you indicate that there are other investors. The mitzva of ma’akeh indeed applies even when the property is co-owned.[3] However, not all agree that this is true when the partners include non-Jews,[4] and the Birur Halacha[5] claims that this doubt is enough reason to not make a beracha in such a case.
Moreover, it appears that before you got involved, there were already people renting apartments in the building. In that case, if there were some Jewish renters, the tenants were obligated in building the ma’akeh.[6] The Minchat Chinuch[7] writes that although renters are obligated, the landlord might also be obligated; however, others say that the Rabbis uprooted the mitzva from the landlord and applied it specifically to the renters. According to the latter opinion, although you could argue that the renters are making you an agent for building the ma’akeh, it is still not obvious that, if there were a beracha, you would be the one to make it.
You further note that non-Jewish workers will be the ones who actually build the new ma’akeh. A non-Jew is not able to serve as a halachic agent, certainly with regard to performing mitzvot on behalf of a Jew.[8] Therefore, your non-Jewish workers’ actions ostensibly cannot fulfill the mitzva on your behalf. It is not that the ma’akeh is invalid and must be redone if built by a non-Jew,[9] as the fact that there is no longer any danger is sufficient. However, a beracha – as well as much of the positive mitzva opportunity[10] – would seem to be missing.
On the other hand, the Machaneh Ephrayim[11] writes that if the non-Jew is one’s salaried worker, we apply the rule that a “the worker’s hand is like the employer’s hand.”[12] This enables the Jewish homeowner to fulfill his mitzva through his non-Jewish employees’ actions, making the beracha appropriate.
However, many Acharonim reject the Machaneh Ephrayim’s thesis. Their main argument is that the rule that a worker is considered like his employer does not apply to a non-Jew’s performance of mitzvot on behalf of his Jewish employer. This is the stronger position.[13] Certainly, there is enough doubt to make a beracha uncalled for in such a case.[14] Furthermore, the Pitchei Teshuva[15] writes that the Machaneh Ephrayim’s thesis applies only to salaried workers and not to contractors (which is probably the status of the workers in the situation you are discussing).
The fact that the fence will replace an existing one raises an interesting question. There is discussion regarding whether a person who switches one mezuza scroll for another should make a new beracha,[16] and there are similar discussions regarding tzitzit and tefillin. There are significant similarities between the cases, but also possible distinctions.[17] The matter may also depend on how long the interim period with no functioning ma’akeh will be or on whether the roof required a ma’akeh before renovations make the roof more accessible.
Given all of these various reasons why a beracha may not be needed, you definitely should not make one.