upgrade 325 to HK stereo

i just bought a 2002 325 with basic sound system. i want to put the HK
system in, but the dealer said that would not be possible as all the
wiring would have to be redone making it not economical.

is this true? from what i'm reading, people are saying they bought the
upgrade kit and get it installed.

any assistance? pointers to documentation (possibly for do it yourself)
or other(better) sound upgrade configurations WITHOUT altering the
deck.

Most will say there are better systems out there than the HK one.