Dude, learn the bus schedules. SD has a decent bus system.
I'd not recommend a car for several reasons - 1st insurance will kill you out there! 2nd SD has a car theft problem, and if your car can be easily stolen or is a desirable one, it will be gone eventually. 3rd, maintenance and repairs will cost a lot for your old beater, and may not be worth it if your car is worth $1000 or $2000. 4th smog checks will be a major pain, and if your car doesn't pass smog, you have to repair it which will cost $$$.
Go ahead and get a good mountain bike (get a lock too!), but I encourage you not to rely on it as your sole transportation this winter. The rainy season is starting soon, and there will be rainy and wet days.
If you plan to live - live there forever or for years and years, then look to purchase a car, but do your homework on theft rates, insurance, going rates for repair, smog check rules, and then decide if it is for you.
A motor scooter is just fine, for the city area and open areas of AlPine, Ramona, or Borago Springs. I think the bikers will laugh at you unless its a Harley Davidson.
nice, all depends on what location you're in and your lifestyle. i haven't been there in 18 months.
car=useable in all weather and ya can take home stuff too.
You are going to need a car here :)