Hi Nicola,
Okay the ADH182L radio you have isn't compatible which you already know.
Below is the AD182H it was made for all MK1 cars with CD Changer compatibility, but is really not as good as the later radios made after 1999 which include more features.
ADH182H
The best Fiat Stereo to get is the AD182H2, it has lots of extra features and you'll be happy with it.
ADH182H2
They also made this model of stereo without the cover flap for the radio, which is alittle better as the flap on some of these can break.
Always remember that these radios are coded so always get the code card with them or do not bid. As they may be locked upon purchase.
The radios on ebay go for as little as £30. So you'll get a bargain.
Regarding your Sony CD Changer. The Fiat CD Compatible Stereos are made to deal with 6CD Auto Changers, and will let you skip with ease. On a 10CD Changer you can skip on the 1st 6CD's but for the next four you can't you need to skip tracks to get to the next cd etc.
However on my stereo the 2nd (2nd one down) but with no flap on it, i don't have numbers to skip CD's, i just have two buttons, up a CD or down a CD. So with that in mind i reckon it'll be ok but don't hold me to that. But i can't see why not.
OK, now if you decided to fit a cd changer it either requires to be Original Equipment Grundig 6Disc Changer or the Sony T69 6Disc Changer. Grundig being the more expensive at £169.
The sony one you can get for £149 Delivered to your door. It looks expensive but because your fitting a Sony CD Changer to a Grundig Fiat Radio it requires a Connect2 adapter, cheapest are £50. So the CD Changer in actual fact is £99 or less if you don't include postage.
Anyway either way your gonna need the Connect2 adapter to connect your 10disc changer or the Sony T69 6Disc one.
Also Fiat will remove your current Radio for free if you ask them nicely.
Fitting the changer isn't easy if your not a hands on person but i managed to do it twice with ease.
If you need any help just ask.
Joske