That looks to be a decent price for that bundle. You might get cheaper by shopping around though. You’ll have the option of using Garmin Basecamp or OS Getamap for doing route planning too.
FWIW, the OSM maps are pretty shite compared with proper Ordnance Survey stuff once you’re out in the hills like you plan to be so you’d definitely need to factor that in.
The G64 doesn’t (AFAIK) support the likes of cadence/heart rate sensors – that may not bother you.
It’s good that it (optionally) runs off AA batteries as you’ll need that ability for multi-day rides.
I currently use an Oregon 600 as I prefer the touch screen interface and it’s a smaller unit without needing all that space for buttons.