If you are after realism, make yourself a testbench and
put a "library" op amp in it from the technology's cell
libraries (if such can be found). Or find a char report for
something at least in a common "node". Realizing that
a full-featured rail-rail op amp may be gross overkill in
fact, as your bandgap-core application has very limited
input and output common mode range of interest,
negligible current throw (tens of uA?) and has to pretty
much just deal with DC, and not make a mess of PSRR.
Now as an output buffer, more op-amp-y goodness may
well be desirable.
You can "idealize" the op amp to see what outside of that
impacts performance and consistency. You can worsen it
to examine "how cheap and crappy can it be and still
make spec"? But for a "typical" report-of-performance
you ought to find a "typical" op amp and that means
picking a platform, first.