Mountaingeering GPS and Sat Nav kind fo dealio with color screen and maps don't really go that well hand in hand.
Color screen and Sat Nav are often quite power hungry...
I would use (and do) use my iPad or smartphone as Sat Nav (with Navigon software, but there are others). And have used a really basic GPS while mountaineering (more than enough if you can mark waypoints and show the current location).
GPS will work anywhere on the globe, it needs to be set to the correct datum and position format to display your location in the correct format to interface with a map, these settings “should” be on every map.
note that if you have only ever used OSGB then you will want to spend an hour or so working with a normal map because northings are not normally straight lines. when you are nearer the poles this begins to matter.
also note that currently declination is POSITIVE 5o 9’ in that part of the world, very important if you only ever work in the UK.
then you just need the map or data. Can’t help there but I’m sure someone will be along to tell you where you can buy it.
I recently bought a Garmin Vista HCx, it was new from Ebay, though they have not made them for a couple of years. You can also pick up used ones for a decent price off Ebay. Its much better than a basic Etrex I had previously which was hopeless. Battery life is very good (25h +), especially if you use lithium batteries. Its relatively easy to use, even with gloves, has a colour screen, takes SD card maps (I just bought one for part of Norway) and picks up sat signals quickly and is small and light and waterproof. There are a number of similar Garmin models of that vintage and many consider them better than the current model range which have large touch screens.
In reply to ERU: Another vote for the Garmin Vista HCx. I have used one for several years in the UK and around the rest of Europe. Battery life is good. A couple of AA batteries generally lasts me all day.
Thanks for all the replies. It seems a simple eTrex 10 (£80) would give me the backup GridRef but the eTrex 30 (£150) does the lot. Garman maps are expensive but there are free solutions online, i.e. Open Street Map.