Well, I see some potential problems with using a charger (thats what you planned, right?).
First, a battery does not really care about "clean" current, so if a charger produces some switching spikes or 60hz hum, the battery doesn't care, but maybe the electronics you attach might. Servers need clean current as well, so here's where the server power supply excels.
Second, once the battery is charged full and the charger stops charging, I don't know if it can fall back into charging mode (=supplying current) when you use electricity from your car, without unplugging it and starting a new charge cycle.
Third, Lead Acid charging is basically CC/CV, and any power supply set to the correct voltage can basically do it. A charger should be able to charge an empty battery in a relatively short time, this is why it will have a slightly higher end voltage, like 14.2-14.5V. If you use your car only very seldom, and it is constantly connected to a power supply with 13.5-13.8V, your battery will be very healthy and surely be charged up after 12h, and due to the high current rating of the power supplies, the voltage will never drop much. But if a charger goes into charge mode each time the voltage drops and ramps it up to 14.5V in the end, this could also reduce the lifespan of your battery by a bit.
Check this:
http://commons.wikimedia.org/wiki/File:Lead-acid_voltage_vs_SOC.PNG
13.5V is certainly enough to charge your battery, just not in a short time.
Many years a go I used a lead acid battery to power my RC airplane chargers at the airfield, and all I ever did to charge them was to connect them to my 13.8V power supply. It was the first ever item I bought on ebay 2001 or so. Aah, memories...
Today I use this:
http://www.rcgroups.com/forums/showthread.php?t=1784667
So, whatever, I can see no real reason why a server power supply should not work in your application. Given that it is cheaper as well, I'd try this route first. Even if it should not work, there's always use for a good 12V power supply.
One thing you might check though: How much quiescent current the PS draws from the battery when not connected to AC. If it's too much, an AC-powered relais between the PS and the car battery might solve that problem. I'll check this out with mine tonight.
I don't regularly use my car as well, and what I did was putting a 10W 24V solar panel on the rear shelf. It doesn't produce enough power to overcharge the battery, but it keeps it halfway topped of all the time, and I have constantly 12.6V when it just sits parked.
