Hahaha, "isolation weight". Ain't that the truth!!! Anyway, lightweight bikes aren't cheap bikes. So what is lightweight to you? For me, my max carry is 20lbs. Light bikes are pricey. Can you carry 25lbs up your walkup? What about 30lbs? That's the first thing to decide because it will determine everything else.
Second question - do you need gears or will a single speed work for you? In other words, will you be riding only flats or will you want to climb bridges and hills?
Third question - road conditions, do you expect to ride lots of potholed rough roads or are they fairly well maintained surfaces?
Fourth question - do you have any thoughts about whether you want a 20" wheel or a 16" wheel? The 20" will be a more comfortable ride on average, but slightly bigger fold and a bit more weight. The 16" wheel will porbably have a rougher ride but will fold more compactly and be a bit lighter weight.
Bike seats (saddles) can easily be changed out and it often takes more than one try to find what's really comfie for you, so don't worry about that right now.
Answer these questions and I think we can point you in a good direction.