+1 on letting a LBS fit your first helmet.
Also, different helmet brands are different shapes, and you may find that some fit you better than others. Going to a shop that has a wide selection is your best bet.
Now, as far as the difference between a $40 LBS helmet and a $200 one... IME it's about a 20% difference in comfort.
I'm not going to spend that much money again.
On your tires, as long as you keep them properly inflated, then gravel shouldn't damage them. The only potential problem is that a lot of areas like that are full of broken glass, roofing nails, fasteners that fell off cars, etc., and grass can make that stuff hard to see. You might want to check it out on foot first and make sure it's reasonably clean.
Speed comes from what you put behind you. - Judi Ketteler