Pets@Home are a bit hit&miss really. Either really nice and helpful and knowledgeable or a bit rubbish.
In your shoes I'd look at 'Cavy cages' for rough measurements and look at C&C cages . You can adjust and customise them.
My boars are very judgey if they have to sleep in their 4'x2' cage (and this is only for winter nights) stuffed with hay to keep them busy.
This year I got them a run for upstairs that's bigger (they tolerate it)
They don't cope with:
damp
extremes of cold or heat - heatstroke can kill them
change of temperature (so not hot house to cold hutch)
draughts - careful with the sliding door if they get a sudden blast of cold. And that they don't get direct sunlight
being alone - they need guinea-pig company
they need to eat pretty constantly, they have the same sort of digestion as a horse, trickle feeder.
hay 24/7. I can't abide the smell of it, but they need it.
Vit C - but it's in their pellets and alot of their veg. Vit C in water can alter the taste and react with metal spouts. You don't know if they drink it or how much. I give Vit C in a Calpol syringe (without the Calopol ) if mine are unwell.
I've tried all sorts of bedding for mine. They like hay outside and soft barley straw in winter as an insulator.
Indoors they have puppy pads under towels and bath mats , with hay in a trug.
If you give them towels or fleeces you need to brush it all well before you wash them or your washing machine gets covered.
Not sure if they'd go for a tunnel and catdoor. Mine are lazy blighters who wouldn't even climb a shallow wide carpeted ramp
Too clean things, there are PetSafe sprays, I use a steam gun and vinegar. (I had to clean our Pig Houses when my GP2 died and we were introducing GP3. That was one of the hardest things, wiping away all the trace of GP2)
There are tons of guinea threads on here , spend lose a few hours scanning through.
If you get your pigs from a Rescue they;ll give you some ideas too. And apart from giving a couple of guineas a new start, you have the reassurance that if anything happens, they will re-claim them.