£75 for fitting that's taking the mickey. Most are ISO fittings so just unplug and plug new one in!!! Usually most difficult bit is getting old one out and finding a conversion plate to mount new one as most holes in dash are not same as standard DIn or double din size head unit. If you can wire a plug you can wire a stereo, just follow instructions and you might need a multimeter (£10 from amazon).
Halfrauds! I agree. Usually it's just a case of unplugging one radio and plugging in the new one. Sometimes the aerial might need an adaptor.
On some Fiats the red and yellow wires need swapping over. It's a common issue.
As you say, it's getting the right radio surround that sometimes isn't that easy.
I looked at a Doblo yesterday and the radio is a single din.
I changed the radio on my Citroen Relay to a hands free Sony MEX-BT2500. It was a great radio. I haven't swapped it over to my Peugeot Boxer due to the surround issue and I would have an issue with finding an adaptor for the steering wheel controls.
I think I could count the times I used hands free while driving on one hand in the Citroen. I don't get many calls. Most communications with customers are by text.
What I did like was being able to stream music from my phone to the radio, but we don't do much driving. Smooth Radio is usually on and that does me.