my opinion is that a PC can never be too powerfull

and since you have it why buy another to use as a car pc..
1. find your cpu's power needs.. (i am wondering how much wattage your current power supply is??)
2. if you can keep it all under 220 watts..then buy a DSATX (expensive but worth every penny.)
3. Avoid inverters (problematicand expensive if you want a good one..or a true SIN inverter...search for that)
4. upgrade your home desktop..
5. using another battery for carpc would lead you to use a battery isolator for charging both battaries..(search for that) and good ones are pricy..
I am powering an AMD sempron 2800+ with many many many stuff connected..
as for heat.. you can always add many fans.. i have 3 in my homemade case.. never run hot and i live in Greece..
just my opinion..