Someone with a true need will be compliant with your rules and won't mind providing documentation.

It would be well worth getting educated on the federal and local laws as well, to be sure you're compliant with the other laws as well.

I would end the tenancy through legally-compliant action as soon as possible. 

If you ran their credit you need to make sure you are compliant with those regulations. 

If someone is still living in a non compliant when you purchase, I think you would be ok as long as you are a good landlord.

All materials are FCRA-compliant and easy to read (which is perfect if you're just starting out!)!

I only cheated on the lower 1/2 bath with a code compliant AAV.We did all the work ourselves so labor was just the fun of doing it.Cost of materials: Main Bath about $2k, 1/2 baths about $225 each from my spreadsheet.We got most of the materials on sale at Lowes: Good quality fiberglass Bath tub $125 (small chip hidden by tile) the toilets $35/each (both non-stock returns), double vanity $175  (side against wall damaged).All the shower stall & floor porcelain tile 75cents/sq ft from auction, Pex, PVC & fittings $185, all elect $100, 1/2 bath vanities $35 each (slight side damage hidden) Lighting $75, all mirrors $100, shower diverter/faucets $275, exhaust fans $30 each on sale. 

The tentant was very compliant and took the listing off that same morning.  

The idea is not entirely dead, but all of the above comments are true in that if you're going to follow the law, you may end up spending just as much to make it compliant.  

No permits but I did have some licensed friends check my work and at least make sure it was sound and code compliant.