Our 03 had lights in the arch already from the previous owner, not sure what brand, but there like 2" puck style halogen lights, they work quite well.
As for the speakers, I wanted to do the exact same thing last year when I upgraded my stereo, but didnt have enough depth. I thought about making spacers, but wasnt sure what to make them out of, and didnt know about this forum to ask all of you guys. Also the issue of trailering with the mooring cover on. When you use a marine speaker, (dumb question) can they be right out in the elements, for instance trailering in a rainstorm, or is they can just get damp from a marine enviornment? In other words are they completly waterproof or more just splashproof?
In either case, running the wires is not to difficult, you need to remove the uphostered pad on each side and there are 5 or 6 inch access holes in the gunwales to get to the arch bolts. I dont know if there was a hole for wiring or the previous guy had to drill one, but it would be that difficult.
As far as putting a hole in the arch, the arch is aluminum, any good hole saw will put a nice hole in the aluminum. Just take your time and run the drill slow.
Also, like I pound in my guys heads at work, anytime you use a hole saw always, always, always drill a 1/4" pilot hole in the center of where you want the hole saw hole, and use the 1/4" drillbit on the hole saw as a pilot only. I have seen it to many times where you use the drillbit on the holesaw to drill the pilot hole and when the drill bit gets thru the material, you can react fast enough, the drillbit goes thru, the holesaw hits the material and bites in, the drillbit breaks and the holesaw skids across the paint leaving a mess. By using the drillbit as a pilot only, lets you control the holesaw so that doesnt happen.
Sorry dont mean to preach, just know from experience

_________________
Jeff
'14 Cruisers 380 Express "Simon Sez"
Merc 8.2 Mag MPI 380
DTS B3X, (x2) Axius Premier
(SOLD)
'08 V318
Merc 350 Mag MPI 300
DTS Seacore, B3 (x2)
