The doorcards do fit, they just have the window switches in them where as the Mk2 has them in the dash. I have some in mine.
 
The seats are direct swap, the rear cards are different in design, but again are a direct swap.

Front door cards are very different, although with a little bit of thought can be made to fit.

The speakers are mounted direct to the door on the mk3 rather than the card, so you'll need a set of mk3 items.

The door release is further forward on a mk3, so at the very least you'll need to mod a set of mk3 cables to replace the mk2 rods, unfortunately you can not just take the complete assembly as the mk2, or at least my 98 used vaccuum on the central locking, where as the mk3 is electric.

The door mirror plates on the mk3 hold tweeters, I used these on my mk2 to make it neater, you may get away with mk2 versions, but if you use the mk3 you will need to remove the tweeter as there is no room for it.

The rest is all pretty easy to suss out.
 
They are the same size, providing you have a 3 door. And yes, it's just the front doorcards you have to make fit.
 
Yes door are the same size physically, just some of the mounting holes are different.

Also, the mk2 door card clips into the window channel at the top, where as when I fitted the mk3 card it sits higher, I suspect the window/seal channel is different on the mk3, Its such a minor detail I'm not worrying about that though.
 
Also, where they meet the dash isn't too bad, considering the dash's are completely differnt.