Buying a dedicated device just for GPS doesn't seem too sensible to me, since mobile phones do the same function (if battery endurance is the issue, you can take an extra portable one - good 10k mAh are around 30€).
Of course there are tons of GPS apps for phone. Best experience I have with ViewRanger app (free). In the app you can buy a special map (1:20.000) of whole Slovenia for around 50€ (sometimes discounts). If you don't need the whole country, you can buy individual parts too. Of course you can use free OpenStreet maps, or anything else you can import to the phone.
I use the app as a portable map that tells me super accurately where I am (this also uses the least battery). Otherwise you can record your route too, use a preset route (the app can start vibrating or alerting with sound if you stray too far from the path). For mushroom hunting I'd mark on the map where I left the car, then use your map reading skills as if you had a paper map with you.