Can Custom Function be used to update multiple fields in multiple lead records at once?

Can Custom Function be used to update multiple fields in multiple lead records at once?

I'd like some advice on whether the following can be automated using Custom Function which requires Enterprise Edition.

Here's the scenario that needs to be automated:

1. User updates status of a task to "Mail Merge Completed".

2. The task update should trigger auto-update of all leads with lead status = "Mail Merge in Progress" to lead status = 1st Correspondence Sent" and change the lead owner on all these records to "xxxxxxx".

I'd also like to know if ZOHO Creator is required or if the script can be written within ZOHO CRM itself.

I'm currently on the Professional Edition but I'm open to upgrading if that's what it takes.

Thank you in advance for any insights you can share.

- Jenny