Which is a smarter financial choice - Buy ready to move-in resale flat at $80k higher or buy the cheaper flat requiring some work with spare $80k to remodel the flat the way we want?

All else is never the same. With 80k a savvy DIY contractor can renovate a double storey house. Most locals are not DIY and if you are those that pay $80 for a hole to be drill in your wall, then , 80k is not much to go round. Do your math with Your ability or laziness in mind.

I would choose to buy the cheaper flat and remodel it myself as I’m pretty sure I can do it the way I want with less than $80K. Some of the fixtures that come included with the $80K worth of Reno may not last long and also doesn’t seem to be inclusive of furniture.

The design of the place will be subjective to your liking. So if you like the design and still well maintained fixtures, then yes.
But another point to note is, do you have the extra $80k in cash? If not, it will be part of mortgage loan which is only 1.8%-2.6% interest per year depends on bank or HDB loans. But if u chose to renovate yourself without own cash, then you have to take renovation loan which is 5% interest per year.

Depends on how long you intend to stay there.

Stay there long term, make room for some budget to do up the place according to your style, if not minor works would suffice