Yea, I have never heard of a SCD unit going bad, they are nearly indestructable, sort of like the saturns. I got the one I use now from an out door flee market in florida, so it sat in 100 degree, 100% humidity conditions until I bought it and it works fine.
I forgot to mention, if you plan on buying it with a genesis, you will probly want to test it before you buy, because used genni's tend to have messed up power sockets, and/or screwed up av outputs.
actually, depends.. I got a dead scd 2 here in which the laser unit went bad. generally the model 2 segacd is much cheaper and less sturdy built than the model 1. on either system, if you treat it well, it will last a long time.
I've got one of each. I prefer my model2, but that's mainly because my model1 frequently has issues with freezing during certain games and not loading with others the first time up. If both worked perfectly, I'd probably choose the model1 because it takes up less shelf space. Right now I only have my model1 set up to play, but that may change if/when I get one of Arakon's multiBIOS thingies.
Just check your potential purchase out carefully, and you should be fine no matter which you choose.