Purchasing a property with tennant already there

The seller needs to provide you:
1) A copy of the lease.
2) A copy of the tenant's application
3) payment history
4) a credit for the amount of the security deposit at closing
5) prorated partial month rent for the end of the month when you close.

If possible, do an estopple letter. That's a letter that reiterates the key points from the lease - rent, deposit, term, notice provisions, residents and anything else you think matters. Get the tenant to sign off. That avoids later conversations like "yeah I now the lease says the rent is $1000, but we verbally agreed to $500."