Oh that washer! That sits (if I recall correctly) above the very top bearing, and provides a bit of tension incase the armature is off balance! Not utterly mission critical if you've got it together by now though, as long as the motor spins nice and smoothly by hand itll be alright
However you do describe the A1 symptom of a burnt out motor! U3480 sounds like a later one, is it 1300w or higher per chance? They use Amtek motors, which are renowned for burning out. Sadly I think a DC04 one is too different I think, although I might be wrong, but having never tried it'll be an expensive gamble if it doesnt! Same with the Turbopower 2, their motors were different, although I wouldn't be fully surprised if they do fit, but itll cost you a turbopower 2 to find out!