there are some differences, as has been pointed out, unfortunately there is little you can do to find out what you are getting before you buy.
ALL current Wii models are softmoddable, the only problem being the previously mentioned D3-2 drive chips. These models can still be softmodded, but will be unable to play backups from DVD-R, you would need to play your games from USB hdd, this is by far the most convenient way anyway, so its no great loss!
So the best thing to do is just buy one and pray! You could buy a second hand model, but then run the risk of maybe someone previously attempting to softmod it and failing or it just being at the end of its life. So make sure you go to a reputable 2nd hand source.
I just want to be able to
play NES, SNES, and Genesis games
play Wii ISO's of games
Stream content off the net (Netflix, Hulu, etc)
How I get there isn't really important to me. Honestly an external HDD would be the easiest solution because I wouldn't have to keep track of or burn a bunch of DVD's.
yes, as far as im aware, the d3-2 machines wil ldo everything except play DVD-R. USB loading will work fine. Emulators will run fine (and can be loaded from the HDD). Not tried streaming from the net, so cant really comment on that one.
As for playing DVD's, again its not something i use my wii for, so im not sure, but would assume that if its a retail DVD it would work, once you have installed the correct software, as a Wii will not play DVD movies without being modded and having software to playback the discs installed.