First, your state realtor licensing agency will have standard purchase contract forms. I've provided the link, presuming you live in Louisiana by your city name provided, in the web reference to this answer. This is the standard form used by most for a purchase/sale transaction.
Second, realize that if a home is not currently offered for sale, and you want to buy it, you're likely going to pay well over market value for the home. When someone isn't selling their home, even if they were thinking about putting it on the market, and they receive an unsolicited offer, you're playing into their hands and will have to pay for the privilege.
There is no pat answer for how much to offer. I also wouldn't necessarily waste time by submitting an offer. Send the owner a letter telling them why it is that you're interested in buying their home and ask them under what circumstances would they be willing to sell it. You need to be sincere in your actions because there are far too many people running around trying to get people to sell their homes below market value and you don't want to be perceived as in that group.
If you really, really, really want the house, and you get an indication from the owner that they might consider selling the home for the right price, see if you can get a feel from them on what that price is. Also, consider, if you really, really want the house, getting a licensed appraiser to do a market evaluation - you can get a licensed appraiser to do a drive by appraisal for probably $250 or so. Money well spent to keep you from offering way too much for the property.
At the very least, make the offer contigent upon a successful inspection of the property, and a satisfactory appraisal report.
In the end, know that the likelihood of paying well over market value is very high, decide if this meets your financial objectives, and be sure there is not a single house on the market that could meet your needs almost as well.