Boar pairs do certainly need more room to keep the peace so I'd get the bigger cage first as introducing an established pair into a new home will trigger hierarchy behaviours all over again - but any pig group benefits from as much space as we can possibly give them (but of course I would say that!) It sounds from your other postings that you've had a traumatic time but as long as you're aiming to pair him with someone you're doing the right thing! Both of your choices are 'right' but I understand that you're trying to do the best for him and what you think he wants. I suspect my George, with the benefit of hindsight, would say he was very happy to trade his nuts for lady companions. You'd have a long wait though, even after the op there would be 6 weeks until he was definitely sterile.
From a 'guinea-pigitarian' point of view it's much easier to rehome the rescued sows and pair up sow babies with mums, sisters etc so the poor boars can get a bit left out and be lonely for longer. There are some forum members with multiple pairs of boars and singleton boar neighbours and it warms the heart that these guys have a dedicated group of boar lovers to cheer their corner so they can be living their best lives. I bet rescues are delighted when someone comes in asking for boars. There's a lot of love to go round.
If you have a long-ish wait for your neutering op you could always try and get him boar-dated in the meantime. If it works out you can cancel the op, if it doesn't you have a fall back position. And he'll get to meet a few more piggies along the way (albeit briefly) which might give you more of an idea about how he'll react to new males. Lockdown is over and the numbers of animals given up to rescues is already increasing...
If you go in for a pet-shop baby double-check the sex yourself. And then check it again!
) If you are worried about liquid try offering cucumber - even if it's just the middle bit smooshed up with fingers.