I would suggest that if you have concerns about the close proximity of a farm you might be better to forget about the site altogether.
There are many different kinds of farm, and each attached farmyard has the potential to cause various kinds of seasonal disruption to you, especialy if you are having concerns already.
For example, if it's a tillage farm, there will be a lot of large machinery and noise and dust in August for about three weeks, but the rest of the year will be relatively peaceful.
A dairy farm will be active from about 6am to 6pm, a milking machine will be running twice a day (not a major noise issue, but if you're sensitive about it you just won't like it.) Possibly cows will cross the road four times a day, so there will be some dirt on the road. Slurry will be spread regularly. Silage will be cut in late may and again in July, so that could mean heavy machinery working into the a.m., twice a year. Calves will be bawling for their milk first thing every morning, and cows will be bawling for their weaned calves right through the night.
A piggery... well they're noisy, and the slurry is fairly foul.
If it's a sheep farm and you have a dog... you'll have to really keep your pet under control, especially at lambing.
Kids love farmyards, there's so much to do and see. All the baby animals and big machines. Of course, a farmyard can also be deadly dangerous for kids. Would your kids know the difference between a big old friendly cow and a tetchy bull?
You could spend time at the site at different times of the day, but as farming is seasonal, what you get today will be different to what you get in September or December.
Another issue you might consider is where your site is place relative to the farm and prevailing winds. That would have a major impact on smells and noise carrying across to your house.
You could always call over and say Hi to the farmer. If you get on well, then obviously that's a good thing, if he/she is not happy to see a house being built so close, then that's another.
But whatever you do, don't build a house close to a farm yard, and then complain about the farming activity and expect it to be changed. That would be like a farmer moving to New York and complaining because there's too much noise, and he can't see the sky for the buildings, and expecting it to be changed. I'm not suggesting that's what you have in mind, but just in case you find yourself thinking.."well, maybe joe'll milk the cows an hour later on Sundays, so we can sleep a little longer" He can't - his cows will get mastitis and suffer.
Sorry for laying it on so thick and negative... I'm just trying to make sure you get an idea of what you could be facing. For some people, farms are just fantastic, for others they're a problem. Which one are you?
(By the way... I'm a farmer)