Welding the crank is when they put a spot of weld every place the crank is pressed together. Keeps it from twisting under high hp. Not too sure what is involved with truing the crank other than making sure the journals and mains are all lined up where they need to go and nothing wobbles out of spec. To weld a crank shouldn't be more than $30. Its not like they are there running a full bead. To true it, again, I have no idea.
To disassemble your engine, get a Clymers manual all the right tools, and about an hour. Putting it back together will be a little longer as it always is longer to put something back together than it is to take it apart.