If you hunt around a little bit, you may be able to find a reasonably low mileage engine from a Crown Victoria, Mercury Marquis, Lincoln Town Car, or other similar cars, ideally from model years 1999-2012.... The later the model, the better, of course.
They typically cost $1000 or less, depending upon the current demand for them in your area.
I think that replacing a engine is completely within the skill level of practicality anyone with a mechanical aptitude, a willingness to learn along the way, a decent collection of hand tools, the ability to acquire larger tools (Engine hoist, jack stands, etc.).
As far as time goes, it really depends... Someone with a little experience, who has replaced a few engines before, with some help from a friend, could probably do the job over a weekend. That is if all of the necessary tools and parts are there, and everything goes smoothly.... It doesn't necessarily always go that way every time though.
If you are pressed for time, like need to start driving the car to work, or something like that, a reliable shop will likely get the car done much faster than a novice.