Chris,
As someone who has been working as a civil engineer in the land development industry in Ontario for over 17 years, there are a few thoughts I have that you may want to consider. Even if the land has sufficient area for a house, there may be a more significant problem trying to fit a septic system on the lands. Being a ten acre agricultural property, I will assume that there is no city sewer available within a reasonable distance. So, woodlots, high groundwater conditions and less pervious soils all could contribute to leaving only a limited area available to place a septic system. I will guess that the groundwater levels are high due to the proximity of the floodplain. The cost of the system starts to increase when materials have to be imported to accomodate the septic system and bring it above the groundwater.
Furthermore, the water supply could be an issue. If there is a well that can accomodate the requirements of the horses and a house, great, but you should likely check into that before closing. Again, on the assumption that there is no municipal water supply available.
Finally, regarding subdividing the land: there are a multitude of issues that could contribute to make this site of limited value to a land developer. Setbacks from wetlands (15-50+m), floodplain requirements, setbacks from ecological features (woodlots, certains types of salamanders, rare trees, butterfly habitat etc.) all make some properties non-viable to subdivide. Even if all the physical/ecological issues could be addressed, the current political climate, the current legislation from the province, the region and the local municipality all make the process very expensive and time consuming and in some cases, total non-starters. Ontario`s "Places to Grow" document limits the ability to subdivide land even more. Some of the projects I have worked on have taken many years and tens (even hundreds) of thousands of dollars in studies to come close to approval.
Land development can be very lucrative, but the due diligence is extremely important for the process in Ontario, especially for the smaller development properties. I have spent many thousands of dollars of my client`s money on investigating lands for them, and and having them drop the deal because they can`t make it work.
Good luck,
Steve