Why not get a
Becker head unit? They have retro-look units (direct replacement for some models), they should at least look alright on a classic car. There are a lot of problems with using an MP3 player like you plan on doing:
1.
Ergonomics. You'll still have to put that somewhere, and youd have to fuss around picking it up off the passenger seat, or you somehow hang it near the dash (like those Discmans hooked up to cassette decks with that line out cable going into the cassette adapter), then chances are unless it's dark out or you have dark tint on the cars, you'll struggle with viewing the screen, on top of the problem with where it is. Then there's the line out cable sticking out which doesn't look good on a collector car either.
2.
System structure/wiring. The way you want to install the system, you're restricted to a single 3.5mm lineout needing to feed a signal for all those speakers and subs. Amps have the proper crossovers, some can even filter the signal for a tweeter, but amps that have line out pass-throughs so you can feed the signal to the amps for all those speakers might set you back a lot, and still get a weaker signal out of the MP3 player to start with - the best audiophile players produce 1.8v AFAIK, but then you need to feed that many amps. Even if you use a crossover prior to the amp, how do you split the signal into all those speakers' and the subwoofers' amp?
There's also this Panasonic head unit that looks like a Marantz CDP, power amplifier with VU meters, and preamp knob mashed together if you have a 2-DIN slot:
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If you're really dead set on using just a single portable player, look for a multi-channel amplifier that can split the signal internally to all four channels, and ditch the extra two sets of speakers. If it's a four channel amp for example you can use two channels on the amp to power one set of component speakers (tweeter+midwoofers) and just use the passive crossover that comes with them, then you bridge the other two channels to power the subwoofer. You don't really need four sets of speakers since if you're listening to a live performance, particularly an acoustic one, all the sounds are coming from the front, not from monitors behind you. Use something like the Fiio X3 or Ibasso DX50 so the line out signal isn't only clean but also a higher voltage than what iPods and Sansas have (while not setting you back as much as the other audiophile players).
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However, if you're willing to really do the research, there
is a way to use a convenient source unit for a car that won't have a visible installed interface, and on the plus side will allow you to use a processor that can correct everything that's wrong in a car cabin, like not being able to afford a Mclaren F1 (ie not sitting smack in the middle; not like you'd actually install a sound system in a car like that if you had one). With smething like an AC inverter, you can use an Android MiniPC in your car, that way technically what you're using is a low power computer as a source, and make sure it has an optical output. You then set this up so you can control it using an Android phone or tablet as a touchscreen display/remote control, and hook up HDDs or better yet to avoid cables get one that has microSD card slots and just be content with 128gb for now. From the Android miniPC you run an optical cable to a (legacy) processor, like an Alpine PXA-H701, or anything that takes an optical audio output (make sure the Android miniPC you get has such an output), then you set the processor settings on it using its own control panel that you can hide somewhere, like in the glove box (modern processors that take an input from the stock receiver's amp just use a laptop that you hook up to it, then access the processor's settings using its own software). If you're handy with fiberglass, you can even integrate the speakers properly (meaning they're angled to minimize reflections on the dash as well as to avoid one or the other being too direct towards the driver) and for the most part seamlessly into the cabin, like so:
