Property, income and capital gain taxes for buyer when home bought under "option to buy" contract?

Seller messed up advertising on house and needs few more months to bring few things up to code before he can sell. Hence as alternative solution seller is offering to rent out home to prospective buyer with an option to buy it once all issues are resolved.

What would be:

1. cost basis for purpose of calculating gains once home buyer will resell home? Would it be one time premium + rent collected + strike price of home?
2. tax basis for property tax purposes in California? would it be strike price or one time premium + rent collected + strike price of home