Times have changed and now heated screens are not such as big a problem as they used to be.
Early GPS units were nowhere near as sensitive as they are now and they also had a limited number channels so it didn't take much to degrade the performance.
The issues these days tends to come mainly from interference from other in car devices such as entertainment systems, blue tooth etc but all in all it's not a major issue.
As for the devices, they all pretty much work the same. Avoid the very cheap devices as build quality often leads to early failures and breakages.
Don't pay too much otherwise you're just wasting money for features you don't need. Think £70/£80 and you won't be far out.
Bill
