There is another major difference between the D40 series and the D80. The D80 has an in-body flash controller that allows you to control an i-TTL flash off camera. With the D40 series, you have to have an SB-800/900, or one of those controller gadgets, to use wireless flash.
If you don't use flash, that isn't a big deal. If you use flash, it is!
If the something in the viewfinder is dirt, not a big deal. If there is something actually loose in the viewfinder, the camera has most likely suffered a serious fall. Same thing with LCD marks... the camera has not been well cared for. A few small nicks or scratches on the body is usually not a bad thing. Stuff in the eyepiece and "rubbed" LCD lean more toward abused.
Although the price is good, I'm thinking there are issues that may bite you in the posterior if you do this.