Simple words are better, right? So why not use the verb end instead of terminate? Here’s what it would look like:
This agreement will [terminate] [end] on October 29, 2011.
Acme may [terminate] [end] this agreement if …
Similarly, you could use the verb buy instead of purchase:
Acme hereby [purchases] [buys] the Shares … .
But aside from ingrained habit, perhaps an obstacle to making these changes is the noun forms of end and buy:
[On termination] [At the end of] [At the ending of] this agreement …
[Purchase] [The buying] of the Shares will …
At the end of this agreement sounds like you’re referring to the back of the contract. And the gerunds ending and buying are a little awkward.
By contrast, the noun forms of terminate and purchase pose no problem.
Related note: See this February 2007 post on the AdamsDrafting blog on termination and expiration.