Its not that tough for a basic install. I've done horn installs on most cars without the extra relay the kits provide, because the amount of current they draw and the duration of that draw (i.e. how long you hold the horn down) are likely such that you probably don't need it. It a little more work if you decide you want the added assurance of the relay since you hafta run some extra wires.
On E46s the best location IMO is the area behind the right foglight. Its the best combination of wiring accessibility and placement for best (loudest) output and weather protection (especially on sedans). People in wetter climes may be better off finding a place in the engine compartment, though.
Work takes about 1.5hrs, a lot of that trying to figure out how to place the trumpets.