They are the loveliest pets. We had a pair and when they both died, we had another pair, so covered a time span of about 14 years in all.
We kept ours indoors in the family room in a cage that was described as suitable for GPs or small rabbits. It was probably 1.5m x 75cm. We used to make them 'houses' out of cardboard boxes, which they would usually end up slowly eating! They like somewhere to hide, as I think in the wild they are tunnel dwellers. We usually filled the 'house' with clean hay and they'd burrow into it and eat it too. They were chatty and sweet and having them inside was a good reminder to keep them nice and clean and not let their bedding get wet.
They are eating, pooing and weeing machines, so do need a fair bit of mucking out. After trying various options, I went with wood shavings and this worked really well for us. The wet bits could be removed daily and clean stuff popped in and then every weekend, I'd do a proper clear out.
The only vet bills was having the male neutered, when we had a male and female together. I got good at toenail trimming, which they weren't huge fans of but again you get better with practice.
In the summer, we had an outdoor pen that they went in, which they absolutely loved. The used to "popcorn" around (a term for GP hopping or jumping) in excitement and they were great little lawn mowers.
They were great for cuddles and would snuggle into a towel on the DC's lap and happily be stroked.
Mine ate Tesco own brand GP mix, hay and fresh salad stuff. In the summer, we would pick fresh grass and dandelion leaves which were their favourite - and completely free!
Our last one lived until it was 8, which is good going for a guinea pig and only died last year. They were all much loved by my DC (and me!).