First of all get a 911 Porche!! hehe 8 )
Anyways, the discussion between Lilliput vs Xenarc has never ended so its really hard to say which is better.
If your using a laptop you dont really need Opus or ITPS, just wire it into cig lighter or something similar, the main thing behind the Opus and ITPS is that it will shutdown the computer when you turn the car off, but since its a laptop and assuming it has ACPI(even old ones do) it will detect that its not getting AC power and turn can be set to turn its self off.
However I would encourage you not to use a 400mhz laptop for this task, Your not going to be happy with its performance at all, playing back a DVD will be really crappy as well. GPS routing also needs a good amount of CPU power especially re-routing if you miss a turn.
That being said you should get a VIA MI or MII board. They are both 1GHz and will be quiet sufficient for your needs. Most people here use those boards, there are a few trying to use the Commell LV671 but that thing costs 400 bucks and you still need to buy the CPU. Anyways - If you go with a VIA MI or MII you will need to buy a PSU(Power Supply Unit). The opus is the nice and clean way to do it, also it will provide more power than the ITPS-70a combo. Both have shutdown controllers and when the car is turned off it will short the power leads and trigger a sleep mode or shutdown.
Since you have a front trunk which is fairly close to the dash I would reccomend putting the computer there unless you want to go thru all the trouble of making it all fit in a double din sized area.
The other option you have for the computer since the trunk is large enough to fit a standard computer in you could build a desktop, or maybe a shuttle style case and put it there giving you alot more processing potential.
Biggest tip I have though is to read read read read and read. This forum has alot of information.