Guineas are gorgeous and generally pretty easy and rewarding to look after. We spent £150 on an indoor cage/stand/hideys/play tunnels, £100 on an outdoor fox-proof run, and probably spend a further £20 a month on average on hay, wood shavings and dry food. For fresh food they can feast quite happily on leftover raw veg or peelings (broccoli stalks, carrot or apple peelings, outer leaves of lettuce - not iceberg lettuce though - celery tops, ends of cucumbers etc) but ours are spoiled and get round lettuces, bags of celery and parsley bought specially for them 
The amount of cage maintenance etc will, as Bonkey says, depend to some extent on whether you have them indoors or outdoors. Ours live in a detached office-cum-utility room in the garden which is brick-built/centrally heated. So they're not inside but they are inside, if that makes sense; essentially they enjoy the benefits of living indoors, and we enjoy the benefits of indoor pigs (we're in and out of the utility room all the time so lots of time to chat to and interact with piggies) but don't have any hay-type guinea smells in the actual house.
this is what we do maintenance-wise:
twice daily (morning and evening) - feed fresh food
once daily (evening) - top up dry food bowl, change water, freshen up bedding, give fresh eating hay, get them out for a cuddle
several times daily - pop in to get things from fridge/get clothes in and out of tumble dryer/say hi to pigs and get conned into giving treats
during nice weather - pigs go out on the lawn in their run
once weekly - full clean out and disinfect cages and replace all shavings/bedding etc.
Agree with getting a guinea savvy vet. Boy-boy and girl-girl pairings both good. Be aware that boar pairs might fight...Sow pairings can still bicker but I think are slightly less likely to have major fall outs. A neutered boar and sow can also make a really good pairing. I had several boar-boar and sow-sow pairings as a child and they never once fell out but we have had a sad experience with our two 18mo boars recently when they had a major fight, resulting in serious injuries (and £130 vet bill) for one pig and they have now had to be permanently separated (so have had to buy another cage/stand). I think we were just unlucky to be honest (they are brothers and had always been together) but it is something to be aware of.