Guess I should add my 02-cents here, since Otis' "transistor radio" is rather well-known.
Key features:
3 Precision Power ("PPI") water-cooled amps (one each for fronts, rear fills, and subs)
MB-Quart 3-ways, front and back
Blaupunk period-correct (post-shaft mount) head unit
Alpine 10-disk CD changer
PPI PAR 225 crossover and PAR 322 effects processor
Dual Optima batteries
Custom-built cooling system
3 8-inch JL Subs in custom-built enclosure
Street Wires dual-output alternator
Three power capacitors (isolated to each amp)
Custom-built dual-point alternator bracket (essential!)
Street Wires distribution and fuse blocks
1-0 gauge wiring (wherever possible)
Dynamat everywhere
Custom plexiglass enclosures, and fiberglass speaker mounts in kick panels and rear deck -- no cutting allowed!!
Upgraded suspension (to handle the extra weight in the trunk)
I'm sure I'm forgetting a few things. Like the miniture radiator in the trunk, with the draw-through fan, for the cooling system. And other things (the more I look, the more I find).
Concerning equipment, I would not recommend going with PPI. In its day, PPI was "the bomb." These days, the company changed hands, and the new company is not very customer-friendly (the tech-line guy is an idiot). Some of the components that I used, for example, are no longer available (the PAR units), so when an internal fuse goes, you're SOL. Also, you might not need a CD changer these days -- some decks will load multiple CDs into the head unit itself. Saves you space, and the extra hook up, fuse, etc.
Street Wires also went through some hard times and ownership changes; in my E-30, I finally gave up on them and just custom-milled all the distribution blocks. On the other hand, JL Audio still is (in my book) hands down the best speaker maker (and still has techs who answer the phone and know what they're talking about). Also, I agree with the comment above, concerning MacIntosh -- looks great, sounds great. It's just a bit tough to find a place that sells it (and will service it).
I had a lot of fun designing and installing the system in Otis. Some purist at the V@V was sitting around in the food area, not knowing that Patti and I were at the next table -- and started ripping into Otis' stereo as "excessive" and "overkill." Well, KMA, and so what? If I drop in a twin turbo, or an M-engine in a car, is that "overkill" or "excessive"? Or is it simply indicative of someone who loves their car, and wants to do something different -- something that no one else has? Some people focus on the engine, some on the trunk -- what's the big deal? I enjoy listening to Gov't Mule, and Storm Large and the Balls (Ladylike -- killer tune) on Otis' system -- and I flat-out simply love the way it turned out. Besides, technically, Otis is Patti's car, and she loves the system.
Anyway, Otis was the 7th system that I have done (the E-30 was the 8th, and it's even better). And in the grand scheme of the car-stereo world, Otis' system is rather mild -- no motorized racks, no fish tanks (yes, someone did a fish tank on a Dodge Ram), no neon lighting, no SP meters, no TV's -- just good, clean, clear, crisp sound through some high-end components. And not every vehicle that I own, has an over-the-top stereo. Hector the Monster Truck has a period-correct (1978-era) AM radio, which we restored. In it's own way, that's cool, also.
So, whatever you do, I recommend (1) careful planning, (2) upgrading the electrical system, (3) soundproofing, (4) no cutting, (5) pay attention to speaker staging and imaging, and (6) fuse the crap out of everything. And above all else, do something different -- with tons of attention to detail.