Doing MQTT on ESPs is easy, I've just switched from the small ESP-01s to ESP12F (on Wemos boards), there's Nick O'Lleary's code for the Arduino IDE that "just works".
For your Android tablet it's a little bit harder but I got https://github.com/eclipse/paho.mqtt.android
working on my mobile phone (subscribing to a MQTT topic that's published by an Arduino with a thermocouple).
MQTT on Raspberries is trivial and that's the piece that glues all of my experiments together.
Publishing NMEA sentences from a GPS receiver is probably about five lines of python to read from gpsd and publish.