The phrase "this is a business" and "training your tenant" come up alot in discussions like this.There is a distinction between what that means said to a tenant and what that means when said to a business partner or peer.You want your tenant to understand that phrase to mean they should pay what is due, when it is due or suffer the consequences stipulated in the contract.

You can probably have the lease stipulate an exorbitant increase when a person or pet not already on the lease moves in even while at the same time disallowing a 5th unrelated person -all lessees being joint and severably liable.This will create a large financial disincentive to violating the lease (and the law) while also giving you recourse should it occur.

Your lease should stipulate a dollar amount that you expect the tenant to absorb for minor plumbing fixes. 

She can ask for a stipulated payment plan through the court, so she can make payments. 

If there's no other extra fees stipulated in the lease, you can't go that route just for being a pain in the butt to collect from.

In both cases, sending a letter saying that if it wasn't resolved in X amout of time, legal action would be taken, solved the problem.It probalby didn't hurt that I wrote a CC on the bottom letter to a family member that is an attorney.Many states/cities have special regulations stipulating that landlords improperly withholding deposits are libable for multiple times the money withheld. 

If your lease does not stipulate timely notice of repairs and the steps tenant needs to give with written notice then you need to get it changed to include that.No legal advice given.   

If you do decide to go 50/50 then include an addendum stipulating that rent is still due to you,landlord, even though you both went half.

So interestingly enough, when I went through training for property maintenance, we defined stipulations of normal wear and tear over 18 months, any residence less than that would be charged for carpet cleaning and minor damages like dings/dents that required more work than paint/spackle.

I don't buy anything with existing tenants for reasons just like this, but if you are going to, you need to have copies of the legal agreements with the tenants before you close, all legal funds held in care of the tenants (i.e. deposits) added to your funds disbursed or placed into escrow with a disinterested third party, with language stipulating access, affidavits of no outstanding mechanic's liens, etc.