If you are on a budget then I would suggest you buy a double din without satnav and get a tomtom or garmin nav for when you need it.
If you can't afford that then just get a good SatNav for £100 or so and make do with the bose until you can.
A friend at work saw my Pioneer DD and was blown away, thought he would do it cheap and get one of these (type) of ebay, he asked me to load some software on it, it was SERIOUSLY rubbish, everything about it was cheap, buttons, mechanism, os, everything, he got a Pioneer and dumped it, lesson learnt!