Since you live in California you need to make sure that you understand the Red/Green sticker issue. All off road vehicles need to be registered in order to ride on any public land and bikes that are 2003 or newer that don't meet the EPA requirements get restricted to only the winter months.
Bottom line: you want to make sure the bike has a green sticker. You will find a lot of used bikes that have been brought in from out of state or for whatever reason never were never registered. My advice is to stay away from these bikes. First off, no registration could mean stolen. Second, while it is possible to properly register them it will take several trips to DMV, which is never fun.
Two stroke vs four stroke is a lot of personal choice. A two stroke will rev up higher but it must be kept revved up in order to deliver decent power. A four stroke will be more forgivng, being able to lug up a hill when you find yourself in the wrong gear where the two stroke will fall on its face if you don't downshift in time.
There are also two catagories of bikes: MX and "off road". The MX bikes are built for race, they will have high performance engines (higher compression ratios) and longer travel suspensions. The off road bikes will be detuned a bit, making them easier to start and generally more forgiving. They also cost a lot less. Some models of off road bikes come equipped with a headlight and even electric start. They also might have a kickstand, which is something I really wish my bike had.
If you think you are going to ride aggressively (not necessarialy fast) then I would recommend a MX bike for the suspension. I ride a YZ-125 and find that it has plenty of power in the flats and only on the occasional hill do I wish I had more power. It will definately get me going faster than I really want to go.
Rod