Is it better to buy more SSD, or is upgrading later a reasonable option?

It depends on the laptop and SSD type. Some laptops, like MacBooks, have their SSDs soldered in and can't be upgraded later. Many laptops these days have a single piece flat bottom and don't have an access door for the SSD. You have to disassemble the laptop to reach the SSD. While this isn't too hard if you're careful, there's a risk of damaging the keyboard and trackpad connectors and ribbons.

Most of the "professional" / "workstation" type laptops are more easily serviceable, either with an access door or just by removing the bottom panel.

I would suggest looking up a disassembly guide for the laptop you want to purchase if this is a particularly desirable feature.

A larger SSD isn't inherently faster than a small one. It depends on the type of memory the SSD  is constructed with, the interface type, and the size of its cache. Filling an SSD close to its maximum capacity, however, will negatively impact its performance.

If you know you want 256 then do it now. It's cheaper. SSDs run faster because there is no physical motion as in regular drives.

Yes. Larger solid-state drives around faster than the smaller ones.