The question with all data gathering is what is the data going to be used for.
this will determine what level of equipment is needed to gather the data.
so for example if you want high altitude overall shots of the whole of a farm like on google earth you need a certain set up.
or you may be better off with a good photographer shooting through the open window of a high winger, with a range of lenses to get shots that can be stitched together to make a large collage that can be printed on say an A0 printer as a large picture or in smaller but higher details pictures.
what sort of detail on the shots do you need, ie what resolution will be key so that is your first question, what do you intend to use the pictures for?
if you want large format, very high res overhead as opposed to oblique shots again, you can do it with a good DSLR and the right lenses if you want to busk it and spend plenty of time post processing, or you can pay for pro mapping shots.
re optimum altitude, yes the amount of coverage for shots depends on the lens fitted and of course altitude and will depend on resolution of camera etc, so lots of high res, stitched together would be my advice for high res, but if the finished picture is say for a large display shot to the public the quality may not be up to it,
if its just for some grubby real estate seller, give em nothing.
if its for internal use, then a stitch job may be good enough, as i say your output needs will determine your input process.