It depends on what kind of "fit" you are talking about. A professional fit is around $100-$200. The kind of "fit" they are speaking of may just mean to be able to stand over and test out the bike. They could throw in other eyeball adjustments too like your seat height and stem length, but I doubt they will put you on a fitting machine.
I can't answer that 2nd question because I don't think it's that simple to answer. Just remember the sizes of the bikes you tried on and liked. Then you can look up the frame geometry at home so you have a better basis to compare it with. When you stand over a bike (feet on the ground and body in front of the seat), you should be able to pull up on the front of the bike and pull the wheel off the ground slightly without, well, squishing yourself.
Having a hilly terrain is even more reason to get a road bike. You'll be in tears trying to climb on a hybrid.
Road bike wheels are a standardized size. The rim width and depth can vary. That and the clearance of the front fork and rear triangle will determine what size tire you can mount. The most common size tires are 23 and 25 mm on a road bike. A lot of people though prefer a 28 mm because of the comfort, durability and potentially decreased rolling resistance (even flat roads are not perfectly flat). Other people can mount a 32 mm tire, but not all bikes and rims will take that.
Cyclocross bikes are pretty much just road bikes that you can ride on rougher and muddier terrain. They have disc brakes, which is a whole different world of braking. They aren't the same as mountain bikes though, and mountain bike tires are not compatible with road bike wheels.
Again, the wheel sizes are standardized to 700mm. The bigger tires refer to the width, but they also extend the height slightly as well. The width and depth of the rim will determine how the tire ultimately inflates. That's why some tires are too big for some wheels; you'll get a 'lightbulb' effect.
This should be all you need:
http://forums.roadbikereview.com/forum.php
A used bike is a great way to break into cycling. If you don't like it, you minimize your loss. If you end up loving it, well, better start putting away some extra money
Edit: one last thing, buy a (very) good helmet and go out of your way to make yourself visible to motorists (bright flashing lights, visible clothing, reflective tape, etc.). You should follow this piece of advice before even considering anything else that I've said.