I was a landlord for about three years and can assure you that it wasn't because I was rich and it wasn't because we have large incomes that can cover two mortgages (unfortunately!). We had a house that was nothing flash, but had to move after my husband got a job transfer. So we found tenants for our house and used the rental income to pay for the rent on the place we were living in.
There would be no way on earth that we could have afforded the $1500 outlay for installing air conditioning for our tenants if they'd asked. They could be the best tenants in the world, but that won't make $1500 appear in our bank account.
To the OP, we had a portable air conditioner that we used in the place we were renting. It did the job, but it was quite noisy. The have to be positioned beside a window so that hot air can be sent outside through a duct. They're quite large also and take up a lot of space. I would think that a window unit would be better if you can get approval for it.
Off the top of my head ours was an Omega Altise 1.5hp and set us back around $750.
We still have it in a cupboard somewhere. I really should put it on eBay. Lol.