I used a drill press to cut plugs from scraps of the wood (garapa in my case).

I'm not sure of the spacing-- but I believe the ladder needs to be in the deep end of the pool. Walk in stairs are typically in the shallow end.

First find out if either is to code where you live. I actually much prefer a glass and metal rail system to cable but that is just me.

I understand that OP needs it to be cheap until closing because this is a lender requirement (not unusual) and if it is fixed before closing, it becomes a fixture and stays with the property even if the buyer doesn't close. CONSIDER: A solution to this would be a rehab type loan rather than the typical purchase loan. Benefit to the buyer is they don't need to resort to expensive hard money type loan as mentioned up thread. Also it gives the buyer the TIME to figure out a permanent solution. The buyer can close right away on a rehab loan. The disadvantage to a rehab loan is that the interest rate is slightly higher than your typical purchase mortgage. Something to explore.