I think you'll find that LCD won't work very well in daylight. My Xenarc has nearly twice the brightness and is still rough in daylight (I didn't get the transflective). I didn't think I would need transflective since I live in the Pacific Northwest and we really don't get that much direct sunlight. Boy was I wrong. Even on overcast days, there is so much glare the screen isn't readable. Luckily this time of year, daylight hours are so short that it's always dark when I'm driving. I know the LR3 has a bit of a cubby for the LCD, which will help, but it also has big windows to let in a lot of light. You can enhance the anti-glare properties of any LCD with some 3M film (instructions on mp3car), but the real culprit is the touch screen layer. That's the part that makes the transflective so expensive.
My next time around, I think I will consider a non-touch interface. I find it difficult to use accurately while driving anyway. The advantage is a cheaper and more non-glare display. And then go for some kind of jog dial interface, such as Mercedes, BMW, and Audi use; in addition to a voice interface.
A lesson I learned on hard drives.... most don't work under 40 deg. F. If you're going to frequently have your rig parked in sub-40 temps, you should look at the Hitachi Endurastar to guarantee operation. Unfortunately, this baby is $200 for 30GB.
You may be able to have the larger cheap drive as a secondary media drive that will become available once the cabin heats up (I really don't know if that will work though).
As a side note, a lot people are worried about vibration damage to hard drives while driving and try to rig up dampening systems. I have just a normal laptop drive with no extra dampening and have taken some pretty hard hits off-road with no ill effects. So this dampening effort doesn't seem necessary IMO.
I'm dubious of a $25 automotive power supply. Does it survive cranking? Does it monitor battery voltage and cut off your system if it gets too low? Will it keep the system in Standby when the car's off? Check out the supplies at mp3car (hope I don't sound like a salesman). Mine's a bit overkill, but you really should make sure it is designed for a vehicle.
I would rethink the bluetooth GPS. You still have to have a cable.... the power cable. A USB GPS also only has 1 cable and will give you less trouble. i.e. the BT GPS I had once required pushing a button for it to connect after being turned on each time. Even though modern GPS units work decently inside the dash or cabin, I did notice a significant improvement putting it on the roof. Especially at stops where small deltas in position were causing the software to spin me around in different directions. Putting it on the roof fixed that completely. It takes a little effort to run the USB cable down the pillar and into the dash, but it's not so bad in the scheme of things and worth it in the long run.
Another thing I did that turned out well is always leaving the GPS powered on. The battery drain is insignificant unless your leaving the truck parked for many days, but it's nice to be able to hop and have an instance position fix.... no waiting!
I also have my system set to go into standby when the car is off , so that it doesn't have to boot or resume from hibernate every time I hop in. This only places a 150mA extra load on the batt. I don't know how long it would take to drain, but I have left it parked for a few days with no sign of trouble. As an extra precaution I did put in an Optima Yellow Top.
I think a car PC is an excellent idea. I especially like the ability of syncing my music library with my home PC over WiFi when I'm in the driveway, and using topographical mapping software for tracking off-road. However, I think it will be difficult to put together a usable system within the budget you've outlined.
