Oof this reply was longer than I planned. But I'm glad you started this thread. I think buying rental properties is a great strategy for making money in permaculture and it is my personal strategy for generating passive income and eventually retiring early. As with any money making opportunity, it is not fool proof, otherwise every fool would be doing it. There are tons of resources online for people interested in real estate investment so I won't repeat too many here. But as you can tell from the other responses, vetting your tenants is critical.
Many permies are dead set on never taking out a loan, and I think that is reasonable. But if you study early retirement and passive income strategies, most literature will recommend the opposite. Spending other people's money is good as long as you are using it make more money for yourself. In my experience its not very difficult to break even with a rental, even early on. After all, rent is often set based in what a mortgage payment would be if you bought that house. But you have to consider the interest on a 30 year loan. If you pay the bare minimum, all of your appreciated value and more goes to the bank. But if you can swing 15 years, while paying extra, you'll be in good shape in most cities.
I'm no expert, but here are some of my personal strategies. I follow the BRRRR method (buy, renovate, rent, refinance, repeat), you'll find plenty of results of you google that.
Buy: I live near Seattle like some other responders. I will not buy within city limits, or even in King County. The rental laws are too strict and the taxes too high. The only way to make money in Seattle is to be a large rental company or a slumlord and I am neither. So look into that before you buy. I look for multi family properties with a decent yard, and as few trees as possible. I want something where I can quickly and cheaply improve both the structure (so I can raise the rent) and the land (I can't improve a mature forest). Your property should serve multiple purposes: income, maybe you live there too, a tree nursery, maybe its a good site for your farm stand, whatever makes sense for you and your area. You want something nearby that you can manage yourself and an excuse to go there often so you can keep a close eye on it and build a relationship with your renters. Also, a good inspector is worth the money, even if you are knowledgeable about what is likely wrong with your house, a detailed inspection report is a good bargaining tool when negotiating the price.
Renovate: A better term would be renter-proof. Assume that your house will be damaged, either maliciously or through regular wear and tear. Improve it accordingly within reason and with durable materials. Is that new hardwood really worth it? Probably not. Fresh paint? Probably yes. Adding square feet or improving the layout is great way to cheaply gain equity (and raise the rent). I don't aim for the highest rent possible, because that limits my renter pool. I look at the median for the area and make my house only slightly better than those. Then, being a permie, you'll probably add some raised bed for your tenants to grow veggies, maybe some expensive cultivar trees where you'll get seed stock and scion wood for your farm. Whatever you are into. That's the fun part.
Now that you've improved the house you can demand higher rent, use that rent as justification to take out a little equity, and use that cash to buy your next property. At least that's the idea.
Keep in mind, buyers don't like trees. By planting chestnuts, walnuts, and hedges on my property, I am feeding myself, my tenants, and the wildlife but I've actually lowered the value of the house. In my opinion, this won't hurt my rent price terribly and I don't ever plan to sell this property so I don't care. But if you're planning to sell this house in 10 years for the gain in equity, this becomes a lot less compatible with permaculture. You'll either have to cut all those things out before you sell (maybe for timber?) or sell at a lower price and shed a few tears when the new owner does it for you.
To address some of your other points, as far as the world ending... yeah... I tend to agree. That idea kept me from buying for years and I really regret it. I could have easily gained a few hundred thousand in equity if I just bought 3 years earlier when I first had the cash. But oh well, you live, you learn. There is a saying among stock traders, "Markets can stay irrational longer than you can stay solvent." So for now, I am living on the idea that even if things go bad in my lifetime, I still have time to make some money before that happens. And I've added one more permaculture property to the world in the meantime. If you've set up an LLC that pays you, you can still make money from your rental properties, then come the end times lose all those mortgaged properties to the bank and still keep your bug out farm assuming you've paid it off
In the short term, with respect to an economic crisis, we'll see. So far housing has only gone up around me. People in quarantine really started to question the value of their shoebox apartment in the city once they started working from home. On top of that, Washington has imposed a moratorium on evictions which has propped up prices as well. Also, democratic presidencies tend to bring fiscal stimulus. I am sitting on my cash for now to see if prices come down before I buy the next property, but I'm skeptical they will come down. Like I said, we'll see.
As far as the housing shortage thing, yeah, I can see why some people think it is immoral to own multiple properties. I don't personally agree. Investing is fickle that way. My 401k probably includes oil companies which I find immoral. But I am not interested in being my own fund manager, so that's the way it is for now. Isn't there some quote about principles being a luxury or something like that? Which leads to my final point.
To me, a permaculture based rental company makes a ton of sense. A trust could buy up property and ensure they are managed ethically. It could provide food right in the city, reducing carbon emissions and food miles. Permies without gobs of cash or real estate knowledge could invest in something that they know fits their ethics. Maybe they could even rent from the company and part of their rent goes towards equity in the company. It gives renters a chance to practice and learn about permaculture. Risk is spread out across multiple investors and properties so we're not all out on our own and we don't lose our butts if things go belly up. At that scale you can get better prices on labor, materials, plants. There is also more opportunity to buy up adjacent properties and design larger systems.
Its probably time I stopped day dreaming about this idea and do it myself...