A simple $300 Garmin GPS Map 76Cx would do, and you could find one for a lot less but you need the charts for here, other models have preloaded charts; learn more on websites about GPS features. You do NOT need a biggie. I sailed here for years with just an old Magellan Blazer, numbers only with waypoint #s inked on charts.
What you'll find are a few things.
1. The fog that other parts of the country get just doesn't happen here. It is RARE for down on the deck fog here and I've been sailing here since 1978. Yes, we had the Cosco Busan, but that was November and it cleared up a few hours later. You will NOT need the GPS to work your way through fog. The fog is at 1,500 feet. Rumors of SF fog are perpetrated by people who don't live here. Oh, I got hit with low fog one June morning a year or two ago, between 7 and 8 am in The Slot, it burned off an hour later, the ferries were reporting 1/4 mile visibility. It does get down on the deck somewhat more often outside the Gate, two or three times a year.
2. After you start sailing out of the same place, you'll find your way back home with your eyes closed. Ya really can't miss SFO, can ya? Landmarks (visual primarily, then with the GPS as waypoints) and comfort with your surroundings increases exponentially as you sail more. Practice triangulation with your charts so you'll always know where you are. Learn traditional navigation before you go all electronic. This is a great place to do just that.
3. Anywhere else you go is so obvious from charts, that you'd do well to maybe get a GPS and deliberately put it away until you're comfortable using your charts. Get to know the #s of the buoys. You have a HUGE channel between you and the Oakland Airport where no one ever sails? Why? 'Cuz the main channel is on the west side and it's kinda outta the way! Nice. If you really like easy sailing, it's a great place to sail around with NO traffic.
4. Learn Ch. 14 VTS, always listen to it, especially when you're north of Hunter's Point.
Save your big boat bucks for a big boat GPS when you get a big boat. Have fun.