Completing cosmetic updates to the interior, painting the whole interior, removing drop ceilings to make the space feel bigger, gutting and renovating one of the bathrooms.

Not including bathrooms updates though.

They "party" (slamming doors, talking ludly, and throwing up in the bathroom) until 5 am.

Try looking into zillow/Redfin and see what your potential rents you can get near the neighborhood by filtering the bedrooms/bathrooms of the intentional property that you are planning to buy.

I recently found a property on Zillow that looks to be in good condition (built 1940s) and has a finished basement that could be rented out as a Mother-In-Law unit (separate entrance, kitchen, and bathroom) while my partner and I live upstairs.

I live in Phx,az and recently friend of mine sold his bank (ex bank of america) literally as is and profiteed $300k  , he purchased it right out cash last year (I think) and it was in real bad shape in a shady part of town,  before him taking possession of the bank  homeless people where living inside it an using the bathroom anywhere in the building, he put maybe less than $5k in just cleaning it up and just a couple of days sold it.Basically the idea is to find super cheap homes in ****** conditions but the price is so cheap that you could still make profit by selling as is, investing as little as possible..

It is a 3bedroom 1 bathroom house and I am wondering if I should invest in another bathroom or leave it as is. 

I bought a 6 bedroom 4 bathroom home now.... https://www.zillow.com/homedetails/5526-Shiloh-Ln-NE-Cedar-R...Ill be buying another one in a year.

For example, if mold was found in the master bathroom ceiling in an occupied home a protocol in that scenario may call for a pack out of the entire master bathroom and bedroom everything within those two rooms would need to be cleaned off-site and then brought back.

The master bedroom, complete with its own bath, is available for $850, while Bedroom 2, with a detached private bathroom, is priced at $800.