qualifying leasor/optionee

I’m trying to do a lease option on one of my rehabs. What is the % of lease option agreement that actually materialize to the leasor/optionee buying the house? i.e. them exercising the option.

The reason I ask is, the applicant’s credit: The wife has a clean credit(although only one open account), but the husband has 12 open accounts(mostly collection accounts, with over $3K overdue). Usually I try to make sure they eventually buy the house, so I try to find people that can clean up their credit easily within the lease period(I like to keep it to 1 year).

Would you let them move in? They earn $3200/mnth, rent is $995 per month.