Currently I'm looking at an area where the house exteriors are in great shape, but the interiors need a safe 20-30k in work to be fully updated to get amazing return on ARV.

And those guys felt they had to justify their expistence by requiring an extra stem wall here, or moving an interior wall back 12" there.  

How throughly do you physically inspect properties that you arent able to gain access to the interior?

No unsightly or unsanitary practice which could undermine the sanitation, health or appearance of the building interior or exterior shall be permitted.And before you ask Brooklyn Au, Yes, I do have a certificate in determining if a practice is unsightly or unsanitary so I am more than qualified to make that assessment.

Interior Painting3.

The interior of the units need paint, some leaking in kitchen and bathroom.

You should have regularly inspected the interior of the property and you would have noticed the subletting and illegal electrical hookup.

By designing it yourself you save money, you can get discounts at a lot of interior finish stores, you can call in some favors from your contractor buddies, etc, etc.

Do not, I repeat, do not, have the first interior picture your buyers see be a toilet. 

The interior is very dated and needs TLC.