-If I'm looking for a PhD, should I go to a less prestigious/cheaper undergrad school and then go to a better graduate/doctoral school?
I don't think so, why would you think that's a good idea? Unless you have lots of money, A LOT of prestigious schools will pay for almost, if not all, of your studies.
-Is biochemistry the major I should take, or is there a better option?
If you were 100% sure you wanted to do biochem, then it's not a bad choice but you could also do biology, general chemistry, biophysics, etc. As long as you take the courses the graduate program you want to get into requires, you're good.
-What should I look for in colleges right now?
They're in an area you like. You'll be spending four - five years there, make sure you like the area.
-Can I use a biochem. degree to get into medicine if I decide to change my career path?
You can use any degree to get into medicine as long as you've taken the courses medical schools require for admittance.