Hello , good choice on guinea-pigs , they are fabulous 
I kept them when I was a child then my DD and I got some when she was 9yo (and entered The Guinea-Pig Spiral , I'll tell you about that later !)
So to answer a few questions . Strap yourself in and prepare to fall in love with the little rodents .
I'll tell you the Good and the Bad ,
is it better to have a standalone cage without a run attached - IMO , it is better to have seperate , Guinea-Pigs , much as I love them are as Thick As Mince so it's better for you to decide if they're going out to graze , they cannot cope with damp . Rabbits otoh should always have choice to go out in the run.
where is the best place to buy hutches? I’ve looked a few places and I’m getting very confused. Also, we’ve seen foxes in our garden twice (over a year ago) so I want to be 100% sure it’s secure Mose hutches are rubbish , but I notice you said you have a big garden..........get a shed for them. Put on a concrete floor to make it undiggable , you can customise it . Mine had a wooden playhouse , my DH made protective bars+chicken wire+mesh layers to prevent anything getting in, we could remove and replace the windows and bolt them in.
I had a rabbit hutch on the floor with a cut-out door and a plastic garden box with doors cut . These were floor level and they could get out onto the shed floor to mosey about .
Guinea-pigs are active as piglets but as adults they will popcorn and have to odd mad half hours but ramps can be a challenge and there's danger of falling .
Piggies really prefer ground level or very shallow ramps .
You'll need indoor accomodation too , they can be out over winter if they are really well protected .We put ours in the small bedroom (no door and no radiator) in big cage or C&C which are brilliant , you can customise it , look up ideas online . You need to line them, they cannot walk on bars . I used a cardboard box and puppy pad + newspaper .
Lots of people use fleece , I liked the Ikea ones but you need a PetBag to stop your machine getting clogged up
are boys or girls generally better it's choice really , we started with two boars then progressed through castrating one and getting sows . A castrated boar and a couple of sows is nice . Lots of boars in Rescue ready neutered . You only neuter to stop breeding , it doesn't make them better tempered .
If you have 2 males they neeed loads of space . Forget the 4'x2' recommendations . Our mixed sex pair were happy enough overnight in a 4x2 but for two boars you need much bigger . Our shed was over 6'x6' floor space .
You can have a castrated boar with as many sows as you like . But only one boar
Or two or more sows in a group
Or two boars . Don't believe the website that says Trio Boars are a good idea , it is tough to keep a boar group happy .
if they’re out in the run, will the burrow out No they won't they aren't equipped with diggy feet but other animals can dig in .
I had rabbit runs from BunnyBusiness ( one was 7'x4' and about hip height ) I could fit a little house in it , put a cover over for shade . We always secured with tent pegs . You need to move the run quite often which is another reason a run+hutch is less practical , you'd need to keep moving it .
Bad Points about guineas:
They need protected against everything - heat , cold, damp(it can kill them) . Predators -foxes, rats, birds of prey , cats dogs
Poisonous plants -buttercups, daisies , bulb flowers
They cannot be sick
They cannot make Vit C but it is in their pellets
They need a fairly varied ,diet . Lots of hay (unlimited ) they sleep , eat, pee/pooh in it
Some things are ok in small amounts ,(dark green veg, can cause bladder stones in excess, fruit can make them looose poohs , and there's a risk of diabetes )
some you cannot give them (includes potato , onion, iceberg letttuce)
They hide illness very well and often when you realise they are ill it is too late .
Then you have The Guinea-Pig Spiral because they need a new cagemate (or two)
Then they might not get on <sigh> so you might end up with seperate cages and maybe a new cagemate
They always need a space to get away from the cagemate if they need to , so 2 doors in a box, 2 water bottles
They are messy , OMG are they messy . Hay gets everywhere and they don't do litter training though IME boars tend to like a little pooh corner .
They don't smell (hay does) but make sure no-one is allergic to hay or pigfur
Good Things :
They are lovely chatty (noisy) greedy friendly little rodents .
They are driven by their bellies so quite bribeable . They are designed to eat pretty constantly so the need for hay 24/7
They don't go for agility (though our biggest boar could jump out of the travel box 
) don't kick or lunge . They do have very sharp teeth but very rarely bite and if they do it is 99% the handlers fault . Either they're scared or startled , in pain or you smell of parsley (and that would be a hopeful gnaw not a full on chomp) One of my rescues was a gnawer but a child might say it bit me !
They don't need vaccines , sows dont need neutered . You will need to find a good guinea-pig vet if they are ill.
And if you have boars (uncastrated) they can get impaction when older , which basically means they're bum muscles get lazy and they get bunged up .
That was an essay .
Hats off to you if you got to the end