I have a hand held GPS for backup nav as well. It is not an aviation GPS but a simple hiking hand held one
(i) Does the gps allow you input your own waypoints? If so, then just put in all the places you want to go, including navaids and go from there. All lats and longs are in ERSA
(ii) The height readout on the GPS is your height above GPS datum. If the datum is WGS84 then it is the height above the WGS 84 spheroid, which in this country ranges from +/- about 200m depending on where you are. Once you factor in varying QNH's then the GPS alt will rarely read the same. Survey ops have the same problem in that the survey alt is normally above or below spheroid and then you have to convert it back to an alt relevant to local QNH.