No you can only download GT:WT DLC, not DLC that used to be available on GH3 on other formats (as the
Wii version didn't have any). Although that wouldn't be a bad idea would it? Neversoft could make them available for GH:WT on
Wii at a reduced price (100
Wii Points each?), or even extend this to all GH1 and GH2 songs - games which were never out on the
Wii - at a further reduced price (50
Wii Points each perhaps, cos they were all covers?). I would gladly use some
Wii points to get some classics from the older games.
As for the guitar, the standard GH3 guitar is compatible with GH:WT, so you could happily buy the Solus game and then get the new guitar at a later date if you wanted. This is what I did cos I already had two from GH3 and GH Aerosmith.