I have been using the iO-Play system for a while now, excellent bit of kit, whilst the Sat Nav units can be used as handsfree the quality and reliability of a Nav unit for calls isn't anywhere near as good as the iO-Play. Have a look at my blog for some details on fitting it and a Nav unit plus powered phone holder to my V70 it might give you some ideas.
As for price, e-bay is the cheaper way to purchase the iO-Play,
This being about the cheapest on their at the moment for a straight purchase rather than bidding.
@anton77 your TomTom One does have Bluetooth and will accept a connection to a mobile phone but only for a data connection to connect to the TT server for information such as Traffic/ Weather/ Buddies etc, bear in mind some of these extra services need a subscription and you will also incur carges via your mobile phone service provider for the data traffic. Something from the TomTom Go range will act as a HandsFree device, but the iO-Play will outperform it every time in my opinion - Mike