Basic Garmin eTrex will do just fine. Although you can do your checks just by reading some distances off the map (e.g. count paces over a full km), you don't really need a GPS.
Did my ML fairly recently too, I'd suggest practising by navigating to small terrain features and evaluating your pacing/timing based on that. Much more like what you'll be assessed on.
It all goes out the window when you're wandering around the Cairngorm plateau in the dark in 2ft of snow anyway… bringing back memories of my assessment there!
EDIT: It's worth paying a wee bit extra for one with an electronic compass. This makes it much easier to follow the GPS to a pre-programmed destination. Obviously, this is a no-no at ML, but if you go on to the higher qualifications, you can/do use a GPS like this.