This has been discussed before, but since there is not search I will tell you what I found when I looked at trying the get Haltech (I ended up going with SCT but that is not your question...)
1. There is no standard wire harness for the Focus offered by Haltech. That means that you would have to create your own wire harness -- not very easy. There was someone trying to put together a group buy a while ago for the harness connectors itself, or you might be able to find one with an old ECU in a junkyard. You would want to use a harness so that you do not have to cut the stock wires (instead, plug in the harness and reroute the wires on the other side). You would then cut have to route each wire in the harness and route it to the appropriate pin on the Haltech. If you are going to keep your ECU stock to control power windows, etc. you need to be careful not to cut any wires that the ECU needs... and many wires will have to go parallel to both the stock ECY and the Haltech. To make matters more difficult, there is not a good wiring diagram for the Focus (at least not that I could find) that will tell you which wire is which ... so you have to trace lines from seneors, pumps, etc. There have been REPORTED successes with Haltech/Focus, but I was not able to get any detailed information from any of them when I looked about 6 months ago, nor was Haltech very helpful when I asked them on their tech support site. This is all very doable if you have a lot of time and are good with car electronics, and espeically if you can find the Focus wiring diagams!
2. Haltech does not offer any base fuel maps for the focus. Their software is supposed to be very easy to use, drag and drop fuel/air modification, etc. BUT without a base map it will be difficult -- it will be easy to blow and engine if you are not very cautious. You will have to develop your base map using computation and know enough to be able to get within 20% or so just using math. Again, if you have the time and skills, not impossible just not for the average joe.
3. If you decide to do this, I would be happy to help out if I can. I would love to have the option of using a standalone for strip use, but I did not have the time to figure all of the stuff above out. I did recently get teh DVD focus manual which has some more wiring information than was on the basic cdrom and I could search through it to see if there is enough info in there...