I wouldn't go for a breed specifically, if rehoming rescues I'd focus on size, age, prior handling, health etc and get something you are going to enjoy learning with rather than the one that looked pretty or sad or.. all the things that a visit to a rehoming centre can lead to you taking home as a mistake!
Few questions/assumptions:
1. Are you sure you only want pets, no ridden, driven or other work? If so go for 10-12 hands high, of native type (ie easier to manage, cheaper to feed, doesn't need specialist care). I personally wouldn't recommend a shetland or purebred welsh or exmoor as a first pony for you, but partbreds and coloured cobby types or other winter hairies that won't cost you a fortune in rugs and supplements and mixes.
2. Age wise I would avoid both youngsters and OAPs as both have specific needs in terms of experience and costs for an oldie can be high once you're into age related normal conditions let alone anything else. Ideal would be somewhere between 8-16, old enough to be mature and have some mileage you can learn from, but not so old you're facing its demise before you get a few years pleasure.
3. Healthwise avoid anything with a history of laminitis, sweet itch or lameness issues. Any of these will cost you in time, money and make life more complicated than a novice needs handle. Avoid also any behavioural issues (biting, kicking, cribbing, windsucking, rearing whether caused by mistreatment or inexperience) and I'm afraid that may rule out a large number of animals at a rehoming centre as the majority are rehomed for reasons of difficulty and cost or age.
There will be a few that pass the above 3 issues and among those look for two that get on, are of similar size/age though not necessarily that close. Geldings are sometimes quieter than mares which can be seasonally hormonal but that isn't always true on either side. Ask questions and take someone more experienced with you if you can. Someone off the board even if you don't have friends with ponies that know what they're looking at (as opposed to just riding).
I have to say do as I advise, not what I did in my day I was terrible for taking on rescues, rehabs, strays, waifs and misc "projects" but I'm more sensible now! One mare I took on at 7 my vet said "only a vet or an idiot would take this on" and he was right and I'm not a vet! And I had plenty experience but even so.. You'd probably be better getting more sheep, but hey, nobody's perfect so go for it and just be as careful to choose with your head as much as with your heart even if you can't make it a totally objective head driven choice the more sense there is, the less stress and cost and regret to deal with!
And I'd rotate them between the 2 one acre paddocks, sheep before or behind depending on their weight/condition. If you can get an early hay or haylage crop off the 8 acre and put sheep on after, you'll save costs, maybe let the ponies run there over winter to rest the small paddocks and reduce winter feeding costs. If the yard is or can be enclosed by gates etc, use it rather than stables as another way of limiting grazing when/if the ponies are too well covered and/or for winter alternative to the big field in really bad weather, with stable doors tied back so they can go in and out, probably they'll share one by choice but may prefer to be out than in.
Major sheep shed/haybarn/stable envy, I've 10 acres and an open handling barn, top half of which lost its roof under snow a couple winters ago! But I've no sheep and more ponies than I recommend you get into!