It sounds like you are purchasing an aquarium from someone who has it already setup? If so, here's waht I would do.
Get a large cooler. Either fill this with water from the tank and add the fish, or use the tank water and baggies, putting the baggies in the cooler. Remove all the gravel and docerations and the filter media, store in a bucket with enough water to remain wet. Dump the rest of the water out. Pack everything over to the new place. Set the stand up, make sure it's level and stable. Give the tank a rinse if needed, then put it in. Add the gravel (don't rinse the gravel, decorations or filter media), and aquascape to your liking. Fill with water that has been treated for chlorine/chloramines, and is close to the temp and pH of the water from teh original setup. Allow water to circulate a bit, then add fish, floating them to balance temps if needed. the cooler usually keeps the water pretty stable, so you should be okay.
Don't worry about saving much of the old water. If it's a healthy planted tank, you'll want to have a couple gallons to start off the micro-fauna population, but otherwise you'd just import waste.
Talk to the current keeper, and find out as much as possible about the tank as you can--size, maintenance routine, age of bulbs, etc. Also get names of fish, if possible, and if it's not "George" and "Plecky".
Welcome aboard!