From what I've heard about the AWD system you'd be fine with the FWD model. Most have said it's not necessarily the truest form of AWD (i have no clue, but it was something to that effect). I would think FWD would do fine in the few times you head into the mountain and it's not like L.A. gets that much rain. MPG would go up (according to Lexus) 2mpg for both city and highway ... a pretty decent increase really.
I think I read about the voice activated not being available on the Lexus website when watching a flash presentation on the Navs, but come to think of it it was probably a tutorial on the OLD Navs prior to the 05 coming out, so I'll bet you're right about the voice activation being on the RX for 05. Are the 05 brochures at the dealerships right now?
From what I have read about the performance package the adjustable height adjustment air suspension really doesn't do a whole lot (of course this is based on people's OPINIONS I have read), but the sequential shift can help make the car feel a little sportier for those who would use it. I personally would just go with the Premium Plus Package myself b/c I would think more about getting the best MPG than having a sporty shifting option. That's just me though. The HIDs have been on the Premium Plus Pkgs in the past, so I would assume that the AFSs would now come with that as well at a higher price for the overall package.
Most people who I've chatted with really like the rain-sensitive wipers, but that'd be up to you. They can be put on separately from the heated seats or packaged together. I would guess most in the LA area are sent with just the rain-sensitve and NOT the heated leather seats.
Not sure if you know this but most Lexuses are just sent out to dealerships in certain configurations and the 05s with the Premium Plus package will have the Navigation system as well and have the 18" rims as well. Otherwise if you want a few different options that they don't send them out in, you have to order and then it would take 3-4 months.