How about one of the amp kits from Communication-Concepts?
Here is a 140W one that requires 1-5W input power:http://www.communication-concepts.com/an762.htm
It is a 160-10 amp that would be great for a QRP rig.
It is a fairly simple build. I am currently building one along with a K5OOR filter board for my K2. I have about $200 in my amp.
They have several other amps as well.