Guinea pigs can be great pets. We have 2. A few things to keep in mind.
Get girls and make sure 100% sure they are both girls!
DO NOT, I repeat DO NOT!!!! Get them from a petshop. Look up on FB if there is a local page. There are several rescue mobs that have FB pages. Look up Guinea pig or Cavies, that should get you what you want in your local/surrounding area. You will be surprised at just how popular they are.
Try to get babies or previously well handled pets. An Adult is anything more then about 3 months old.
They need to be handled a lot to maintain enjoyable pets. Keep them either inside or in a high traffic area of the yard.
They need LOTS of access to water. They will die easily on a hot day so keep this in mind over summer and either inside in the air con on hot days or make sure you put some frozen bricks in their cage to keep them cool. On the flip side, when its cooler, if they get wet, say they are outside and it rains while they are uncovered, they will can get sick and die from a chill also.
Boys stink.
Girls... just make sure they are both girls lol, they will fall pregnant if there is a boy.
They are generally clean animals who wont pee on you if you listen to their cues. At least mine actively speak up with noise and behaviour if they need to be back on the grass.
Handled well, they do not bite, or scratch etc. Handled incorrectly, they will panic and most likely scratch the person handling them.
When they are happy and content they "pop corn" around the cage, which is very cute.

As for holidaying.. Get them when you return. Maybe the cage and accessories could be on xmas day. If you find a local breeder, chances are they will be happy to baby sit while your away.
Cost wise, other then the cage set up. They will set you back between $10 - $30 each. Find a good stock feed place and they will usually do up a guinea pig mix for you. Pet shop or supermarket pellets isnt a good diet for them. They like grass and grains. Fruit as treats, veg as often as possible, especially high vit c foods. Mine love Capsicum, the husk and hair of corn, bok choy etc.

HTHs