What are your needs? Do you sit in your car for long periods of time, wishing you had a film to watch? Do you play games or surf the net and wish you could do it from your car?
The In-Car PC is a good idea, as it gives you all the storage you want, movies, DVD's, Sat Nav, internet, and a whole host of other things. But you have to be honest and ask yourself if you'll actually benefit from all this?
If you just want the ability to watch films on the odd occasion, but want loads of music on board, there are other options....
See if you can find a Double-DIN dash fascia for the Leon. They are a couple of hundred pounds new from Seat, but occasionally crop up second hand. These are the dash fascias that are used with the standard fit Sat Nav option. Fitting one of these will allow you to fit a Double-DIN sized head unit of your choice, and the choice of these is getting larger all the time.
Alpine, Kenwood, JVC and Pioneer all make Double-DIN sized screens. Imagine a pop-out screen that's not pop out. This one is the Alpine option, as an example of what I'm blabbering on about....
Now this plays CD's, MP3, DVD's, MP3's from a DVD (so 4+GB worth of music!)... as well as being a damn good head unit. You can also add on Bluetooth hands free, navigation, TV Tuner, iPod connectivity and a few other bits and bobs should you wish.
They aren't cheap... but aren't far off the price of a good spec In-Car PC and screen.
If you want those options, but don't want the Double-DIN idea, then just look at a normal pop-out motorised screen instead.
That aside, you still have other (cheaper) options. If watching films is a priority, then look at JVC KD-AVX2 that is a normal sized head unit, but plays all the same things as the Alpine I mentioned above. This also has a small 3.5" widescreen TFT built into it. So not great for turning your car into a cinema, but ideal for the occasions where you're stuck in the car waiting for someone or something.
Other option is as you mention... a head unit and an iPod with the necessary lead.