One idea was email monthly rent invoice slip with a prepaid envelop for the tenants to print and mail or just mail back.

I always suggest to my clients to get pre-qualified by both.You can ask up to 4% of closing costs, prepaids, and reserves be covered by the seller.

If you pre-paid, then that may be another story.After all of my years in the business, I don't have a good eviction attorney yet. 

If it's not in the lease, you might have problems enforcing the guy to do the maintenance, but the discounted rent you will have to honor.Close in the beginning of the month so the seller will have to give you credit for rent at closing (rent is due on the 1st, close on the 5th and have title company credit you for 25 days of pre-paid rent -- it shouldn't be your problem that the rent hasn't been paid on time, make it the seller's problem to collect it).

(b) If the tenant abandons the dwelling unit, the landlord may send notice to the tenant at the tenant’s last-known address both by regular mail, postage prepaid, and by certified mail, return receipt requested, stating that (i) the landlord has reason to believe that the tenant has abandoned the dwelling unit, (ii) the landlord intends to reenter and take possession of the dwelling unit unless the tenant contacts the landlord within ten days of receipt of the notice, (iii) if the tenant does not timely contact the landlord, the landlord intends to remove any possessions and personal effects remaining in the dwelling unit and to re-rent the premises, and (iv) if the tenant does not reclaim such possessions and personal effects within thirty days after the notice, the landlord will dispose of them as permitted by this Section.

I suggest you make a contract where the seller pays for an appraisal and closing costs except for your pre-paids.

The Minimum Required Funds (“MRF”) are funds you must have available to pay costs associated with the purchase of the property, pre-paid expenses at closing and a reserve for costs once you close.

They allow as much as 3% of the closing costs and prepaid taxes to be paid by the seller.

[removed, not relevant]... the landlord shall furnish the tenant, by personal delivery or by first-class mail, postage prepaid, a copy of an itemized statement...My understanding of the above is the landlord has a full 21 days before sending the statement by mail.

@Mike ReynoldsTexas has zero restrictions at all with pre-paid rent or depositsYou can literally put it all on Justify to win the Triple Crown if you wantDisclaimer:  NO that is not sound financial advise !