of course, all these answers are gonna be IMO, but;
1 - depends on what your budget is. I remember i found a primiered bumper for me (06-07) for like 80-90 bucks. but ya know, gotta get it painted and such.
2 - i think if you have to replace the grille/get the grille if you convert. i believe the 03-05 have the grille and bumper together? ( i don't know for sure, just from what i've looked at ) since the 06-07 bumper and grille are different.
3 - you can get them from honda or any plates place. i wouldn't drill into the bumper. the oem ones have a bracket that attached underneath.
4 - and the lip, all depends on how low you go and what not. it def will affect how you drive, like going up and down driveways and entrances and over speed bumps. like me, if i go over certain speed bumps straight, my frame will hit it. lol . but it's your call.