Will I experience any operational difficulties with a top heavy car?

Depends on the state of your track - if you lay perfect track with
transition curves it may not be an issue, but top weight makes the car
unstable on poorly laid industrial tracks or authentically decrepit
short line

Here's a couple of thing that have worked for me

1 Cast the floor in white metal using the kit parts as a master and
high temp RTV for the mould. (Only works if you have a tolerant
domestic authority or your own workshop where you can play with hot
metal). The metal casting will tend to shrink a little so it won't be
a perfect match

2. Check the small suppliers for white metal or pewter occupants I
used some OO scale cows from Langley models mixed in with plastic cows

Aidrian