Buy economy first then upgrade or MUA

I’m looking to book a couple of tickets from LAX to HKG in the summer. I have no status with United. I have a couple of questions:

1. I want to try to get Polaris. Should I buy economy first then upgrade, or book MUA right away? If MUA, for the dates that I’m looking, only “upgrade waitlist” is available but I heard this segment is one the easier to clear for Polaris

2. If it’s former, do I just buy the regular economy (non refundable) or economy (fully refundable) if I want to upgrade after? Understand that as long as it’s not basic economy. Can I do MUA after I bought economy?

Thanks!