either with a cash out refi or to rent for extra cashflow? 

Then add in closing costs, etc. and you might walk away with a few extra bucks to plant new flowers at the 8-unit.

He wants to buy a new car and put it up for rent so he can make some extra money.

I know a 1031 is still possible under the installment sale, but honestly I don't see the seller going for this with its extra complications.

Then you’d want to budget for providing minimum service to condition that extra “interior” square footage.

The extra $5000 doesn't seem worth it.

There are shortages on everything right now and anything custom is likely to take extra time.

The report from Bigger Pockets seems like a good "base hit" to me, but I'm spending $57,500 just to make an extra $400/mo.

And it was used to make Crystal but again now mostly lead free crystal which as far as I can tell is just extra clear glass.

The rental market is hot right now so it probably wont matter, but in normal times, I like to pay a bit extra to get a 2-tone paint (tan/white trim) to make the unit stand out a bit, but again, just preference - I don't think either get you more in rent.