Can I tell you about guinea-pigs? I could write books about them and my guinea-pigs would turn round a disprove every word 
Consider what they Need and if you you can provide this , then start planning.
They need company. Other guinea-pigs. Most places (Rescues , breeders, repuatable petshops) won't let you take one pig unless it's to match up.
They need enough space (especially boars) and it needs to be secure . Bad weather, rain,damp,wind, extremes of temperature. High temperatures, sun.
They need you to be aware of their health, they will hide things. They are prey animals so if they're ill, they keep it to themselves. I knew my pig was ill by the way he was sitting, he died two days later, even after seeing the vet. If he'd been left in the hutch and not looked at, it would've been missed.
You need to check they're eating, drinking (though mine don't) poohing, getting along. They have a Pecking Order, but one can be a bully.
Foxes,cats,dogs,birds of prey, all predators.
They don't dig, don't kick like a rabbit, don't bite and have quite stocky bodies so not really designed to defend themselves.
Ours have a wooden playhouse that my DH altered . We have windows (two fixed plastic ones and two we can take out for summer. Inside is re-inforced with metal bars, wire fencing and flymesh.
A fan, a light, a heater. Snugglepad.
Ours do come in at night for winter (from Guy Fawkes till Feb/March depending ).
Hay gets everywhere our small bedroom (the Guinea Room) and all down the hall/stairs has hay scraps .
We bought a big rabbit run from eBay (I'll find the site, I think Bunny Business) it pegs down with tent pegs. A small run when our boy was neutered.
Indoor cage is 4'x2' ( too small for 3 hogs really but it's just for night time sleeping. We fill with hay, they're happy). I got this from The Range, Jolleys have nice ones too. It's a plastic base, wire top.
Most cages in Well Known Pet Shops are very small, or the hutches can be flimsy. I wouldn't trust them unless they were in a secure shed.
Minimum 4'x2' for 2 pigs, 5'x3' for 3.
Boars need as much as you can give.
They like a run-around area ( run indoors or outside when it's dry and warm enough)
The Pighouse is just under 6'x6' (I measured it when I got my new pigs), has a wooden haybox that they sleep in.
I kept pigs from 9-22yo.
My DD wanted pigs when she was 9yo, she wanted boars.
We got our 1yo Rescue brothers. Lovely pigs, bit narky if they didn't have enough room (and fighty with a coriander overdose)
When GP2 died, we got a tiny 6 week boar to match. Rescues have all ages and personalities. We needed a very young boy (not bothered about colour) our adult boar chose.
When the piglet was 1yo, the older boar died. We decided to get females (same Rescue) , we got two bolshy sows who had been seperated from their piglets.
Boar was neutered (obviously not without risk and cost about £58 , we kept them in 'chat' distance till he was sterile, they bonded.
My little sow (always the little ones) is the stroppy one of the group, but they are all lovely.
They are hard work. My DD and I share (she cleans the night cage, I do the Pighouse, feed them , put them out, bring them in)
We clean both houses daily (in the Pighouse, just the haybox daily, main house weekly at present). They don't smell, (hay does more than pigs), but they don't thrive in dirty or damp bedding. And 3 adults pee, oh yes
We love our pigs. If you buy some, go to a Rescue. They can match you up and you get a back-up. Any reason you can't keep them, they will re-adopt them.
Check your DD isn't allergic to their fur or hay, my DD (now 12.6) was fine with her first two boars, allergic to the 3rd boy. He's a Rex (coarse coat) if she kisses him she'll get a reddish rash.
Good Luck