I have a few questions about frame repair, seat adjustments, and rims.
Yes, it is possible to powder coat a frame.  They most likely wouldn't put new decals on.  You can usually find replacement decals online, shouldn't be too hard.  The powder coating process is a bit involved, and probably will not be cost-effective.  But if you want to spend the money, it's definitely possible.  
First, let's get this right.  You're looking at "wheels", not "rims".  Sorry to be pedantic, but the terminology does matter here.  Single speed means what it says.  It has spacing/cassette body/freewheel threads for a single speed set-up.  Sometimes this can be adjusted(at great effort) to work for multi-speed systems, but more often it can't.  
Had a hard time following this at first.  I think you're talking about a seatpost clamp.  First, there's nothing "classic" about one that takes an allen wrench.  Those are a bit more secure(if your bike is locked up somewhere, people will be less likely to walk up and steal your seat), and road bikers tend to change their seat height less frequently.  There are Quick Release options available for most sizes, as long as the clamp isn't integrated into the frame.