Capital gains aren't that big of an issue, because the current long-term CGT rate in the US is zero. There is a withholding tax requirement in the US the purchaser (not you, the person who buys it from you when you sell it) has to comply with on Form 8288, however (a) they don't have to do it if you can show them there is no tax (which there isn't currently) and (b) they don't have to do it if the value of the property is less than $300,000 and they intend to live in it for at least the next two years.
However - because real property is considered "effectively connected with a US trade or business", you still have to file a 1040NR and a schedule D with the IRS, even though it will be covered with zeroes.
So basically CGT other than that piece of paper will work the same as it would if the property was located in Canada - unless the State has a capital gains tax and many do. But you can claim a foreign tax credit for that on T2036.
As far as mortgages go it is getting more and more difficult to get them, I spoke to a mortgage broker at Bank of America in February as I recall and she told me they did still do them for Canadians but she knew that BoA was the only large commercial bank still doing them in the US and that it wouldn't surprise her if they stopped.
If you really want the answer, talk to Bank of America, they will tell you what the down payment will be, I think she said 35% actually not 25%, might not be remembering that correctly though.
As far as dealing with rental income, I suggest you have a read of CRA P151, which is a bit out-of-date but will give you the general gist of it.
http://www.cra-arc.gc.ca/E/pub/tg/p151/README.html