Seriously, you often need some non-volatile RAM or EEPROM for configuration or machine settings. Some 8051s have built-in EEPROM. Others can write to some flash pages.

Most modern chips can sleep and retain SRAM memory from a coin-cell.

Yes. You can use up 2 ports on an external memory bus.
A suitable 8051 variant can do the whole job in a single chip. And you require zero code changes.

Without any quantitative information, you can't look up any parametric tables. And we have no idea whether you want / need 1kB or 1MB of SRAM.

David.