Even though the proposals seem to be very similar, the best option would be the second form: We propose to install and landscape a 100-gallon pond at the front of the property. The pond will feature a 30-gallon per minute pump and fountain.
A business proposal should be well written and direct, not leaving room for doubts and misinterpretations. The person offering the service should make the offer clear, stating what will be done and how it will be done.
Using a direct structure for the sentences and Active Voice - instead of Passive Voice - helps making it simple and less likely for the reader to misunderstand who will do the job and how it will happen. That is precisely what happens in the option above: it mentions, who (we), action (install and landscape), what (a 100-gallon pond), where (at the front of the property). The second sentence begins by using the word "pond" again to make it clear that it will be the pond installed that will present certain features.
It's a direct business proposal that will most certainly not be misunderstood.