XXX is:
I am faced with the choice of "slots"
Who can advise from personal experience?
I go to Lenin. Long time of autonomous work
Budget up to $400, GPS accuracy receiver.

YYY :
Take the Bundess folded blade:
It fits into the budget (for 400 you can buy from dozens of blades).
Excellent time of autonomous work - in six months never charged.
However, the accuracy of the allocation of coordinates is a little crumbling - only by the sun, having stumbled in the sand, knowing the exact time, you can more or less calculate the latitude. If you magnetize and drop into the water in the pad (not included), then you can find out the direction to the north.

and ZZZ:
Clothing is no longer in fashion.
After use, the sand is stuck.