There are threads here and on other boards for other vehicles on the Tech II units. I've not read a post about one messing up the vehicle. Some about the units themselves not working, though. That said, those people wouldn't be in business long if they don't at least get you a working unit. Places like alibaba have dispute resolution through the site, if needed. You may have issues getting any type of support down the road, but you should be able to get a working unit at the beginning.
You can likely get everything you need for under $300. Assuming you already have an old laptop with a serial port that you can use for the programming side, as necessary (doesn't happen very often).
Alternatively, you may want to check into the MDI units. They will support the newer model years that the Tech II doesn't as well as being backwards compatible. I have no experience with those, sorry I can't help further. If I was looking for a unit now, I would at least consider an MDI.
Good luck.