Adjoining Land Potential Purchase – Advice?

I am in the process of finalising a purchase for a house on 817sq metres, which I am planning on developing in the next 2-3 years. Currently this is a 3 unit site.

The back of this property abutts the back end of another property. The owner is looking to sell the back part of their property, measuring approximately 300 square metres, which is landlocked. They have approached the agent who is selling the property that I am purchasing, with the intent of offering it to me. They will pay for all the necessary costs to subdivide.

The combined block would potentially allow me to build four townhouses and also increase the average size (sq metres) per unit site.

The purchase price for the original property is $394,000 and there has been some mention of $120-$140k for the additional land. All other things being equal, it would be preferable to purchase at a price which lowers the unit site cost for the four townhouses.

A couple issues though come to mind about the potential purchase of this additional land

*4 townhouses become a commercial development, requiring a pre-sale. The intention for the redevelopment is not to sell
* Greater risks to objections to the development, given the new combined land would abutt onto six different properties instead of four now
* Difficulty in assessing the fair market value of this 300 sqm given the circumstances
* There is a sewer main right at the back of my property, so am not sure how this would affect any development for four

Has anyone else done something similar? What are some of the pitfalls or considerations in contemplating the additional land acquisition (financing aside). Any help would be greatly appreciated

Woodsman