Talk to them when you renew the leases and be prepared for vacancies if you change a lot of things or aren't flexible (ie on how they pay rent). 

How much are you prepared to invest?

I am torn between pointing the rocks with hydraulic cement and dry locking versus having an interior waterproofing system installed.

They charge the Landlord a leasing fee equal to one month of rent and then they charge the Tenant a "Lease Preparation" fee or something similar.

I do think there can be value in having someone prepare your taxes, but you want to find someone who a)understands real estate investing, and b) is willing to help you with planning and strategies as your business grows. 

Do you all use a lock box?

There are many areas for STRs in SoCal I would just make sure two things when you get that partnership locked in you should have the location picked out and price point.

I'd recommend having a lawyer prepare the LOI.

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This implies if the LL takes one month to place a tenant and a couple hundred dollars doing the always tenant turnover items (for example we always change lock cylinders, shampoo carpets, chemically treat the drains, deoderize/disinfect including use of ozoner).