MITAC MIOMAP M701
It's a cellphone that will take an ordinary SIM chip - not a mini, not a micro, the ordinary one.
It runs WindowsCE which is very light and stable.
Very good touch screen with built-in stylus.
Charges off the ubiquitous mini-USB (the earlier kind)
Replaceable battery (which is nice these days)
Standard size SD-RAM card up to 8GB (which is plenty for a 15 y.o. phone)
It has true GPS capability (not "A-GPS" which is wifi/data assisted)
It has a jack to accommodate an external antenna (i.e. on your pickup or quad or trailbike... canoe?
)
It comes standard with MIOMAP GPS Navigator, but it will happily take TOMTOM Navigator 5 or 6.
It will work without a SIM chip, BUT if you slip in an extinct (disconnected) SIM
it will enable you to call 911 whereven, whenever,
because it will lock onto any carrier signal it will find.
Which is great because often you may get the "no signal" message on screen not because "no signal" but because you own carrier's signal is lost.
Maybe there's somebody else but you? No signal.
Not so with this baby.
I have two with pouches, one with TOMTOM Navigator and a 10 y.o. map of the 48 lower US, the other with Europe.
I highly recommend it - you can find accessories on AliExpress or AliBaba, and with some luch a complete system on eBay.
https://www.ebay.co.uk/itm/Mitac-Mio-A701-DIgiwalker-batteries-memory-map-map...GB£40 (=~US$50) + s/h