Answers in confused order.....
The maps will depend on the package you buy. There are basic (mapless), Navigator (SatNav mapping on 605/705/800 and
Ordnance survey on the 800)
All of these will take up free OSM street / cycle maps with full search and routing
The actual choice will come down to what you want this for. If it is a cycle computer then these are great, but expensive. The Briton Ryder 50 is £100 cheaper than the Garmin 705 and reportedly performs similarly. Be aware though that if you buy the basic and wish to expand then Garmin 705 models only support Garmin HRM and Cadence,
However if it is just for navigation then an ETrex Vista HCx or similar will use OSM mapping and is only about £120
The best site to read about what these are capable of is
Frank Kinlan's excellent blog. It does seem to be down at the moment though