Automate apartment entry

Better still, install a receiver in the apartment which is compatible to your vehicles system then use the car's transmitter's auxiliary channel to open your apartment.

How heavy/big is the lever in the apartment? How much "travel" is required to pull the lever? A door lock actuator might work better than a trunk release solenoid.

The brain itself would function on as little as 500 mA to 1 amp but the actuator would need more.  For power, get a small power supply, 5 amps should suffice. Again, power is relative to the work being done.

The guy I work with has a Viper SST security remote start combo in his car and a Viper security mated to an internet based vehicle tracking device for his home. This way he can monitor his home from the web and get alerts on his cell if anyone enters his home. The whole thing works off a battery and charger so it works even in power failures. He'll get paged even if thieves cut the telephone lines. The whole thing comes out cheaper and is more effective than if he'd purchased a home security system. Genious.

Go to any electronics store and see what they have for a 5 amp power supply which will convert the house main a.c. to 12 volts d.c.

The computer supply would work and there many sites that show the required mod such as this http://www.nmia.com/~tandberg/LinkedPages/SailplaneArticles/PowerSupplyATX_03.htm