I use a Garmin like this
nüvi 3598LMTD | Garmin
I can't recommend it enough - absolutely brilliant with free updates for life, great graphics, speed warnings, camera warnings, all sorts.
We have a Brodit mount in both the Subaru and he van (we use it in both) which is definitely the best way to mount a portable satnav.
They do a version for the Transporter/normal dash and the Cal/Velle comfort dash (make sure you choose the right one!!).
Of course there's the wire "issue" but it doesn't worry us at all.
We had built in satnav in the old Cali and we have it in our BMW- it's okay but dates SO quickly, updates are a pain and usually expensive (£200+ in the BMW), most don't do camera warnings or speed (ours tells you the speed limit and gives you your GPS speed) and so on.
I also appreciate phone nav is good but around here, I've had too many cases of not getting enough phone signal and so having no nav. A portable satnav doesn't need phone reception.
I'm not paying for factory nav again - although the new M140i has it for "free" now so I'd probably accept that...
With a Brodit mount, it needs to match the satnav and the vehicle (you buy it in two parts) and it's about £40 but worth it IMO.