I might suggest you could go a hell of a lot lower, if you can find a 
DOS-based image viewer.

You might try the swap meets... I've seen (and bought) 486-class lappies 
whose primary problems were bad screen hinges (not really relevant if you're 
rebuilding into a frame) and-or bad batteries for like 10 USD or less.

Nothing in your task couldn't be done with a 486-40 (common lappie speed), 
with 8Mb of memory and a 200Mb hard disc (or why not a CF card in an adaptor?)