You'll learn a ton :)

$500,000 after taxes is likely close to $325,000(maybe more or maybe less).Assuming cash-flow from rentals is offset by depreciation, you would need $325,000 of annual cash-flow from the rentals.If you can get $200 / door / monthYou would need 135 doorsYou can work on this type of math to figure out how much rentals you need.Be mindful that rentals require some type of work.

A buddy has a 5 unit building he needs to sell at 475k.

This forces people to relocate.

Appreciate if anyone can provide a rundown of costs to expect with this type of transaction?

You forgot to put your location in your profile.

You're just going to need to find a couple of folks on the ground that you trust.

However as Texas only allows a HELOC on your Primary Residence, I don’t want to lose this facility if I move to a new place (and I want to comply with the rules!).

I have been interview these folks and almost down to the right folks.What I am seeking from the community is experience from those that have done this and have been successful but also those that have pitfalls to learn from those to either not make the same mistake, or do what works. 

If you do more than one unit this way make sure you don't need a boarding house license in your town.