Great tips so far.
I am in N.E. OK, and the rainfall shouldn't be a problem I don't think. Most of the soil around here I believe has VERY good clay content, so it shouldn't seap...shouldn't obviously being the keyword.
I also understand it's hard to say from my general post, I was just trying to get an idea of if it could be done. (not so much legally per se, just that yes it could be done, and not flood etc)
I am quite honestly looking more at 10 acres when we build our house (and then pond), but some of the differences in the properties themselves (not to mention the $ difference) makes me just want to stick with 5 acres. I have found great 10 acre lots with crap for building a pond, but great for horses. :p

What else? I plan on having a local NCRS come out before I buy the land to see if a pond is feasible, and how big I could go.
Up here, every 6th or so house has a pond, and the ones I have fished/seen/heard about, don't seem to have much watershed. One lake I fish locally is 110 acres, and it looks like there is only 20 acres TOTAL for the watershed.