Well, I was thinking about using a temp sensor combined with arduino to control the outside fan, on the hot side. When it reaches above 50C it would turn on.
Also when the "fridge" reaches it's set temperature(temp sensor inside too), the peltier and fans are turned off. And turned on again when needed.
Anyhow, might be enough to use the 50W peliter. I dunno how much 50W is, when cooling down a 50x30CM "box" with stuff inside.
Or maybe the 136W would make a freezer? 
Or... ebay got a 400W peliter, but then again I might just need a watercooler to keep it alive