As long as you can get easy access to the hull, installing the various auxiliary packs is not usually a problem at any time (indeed, some of us have come up with systems for easy removal for subsequent maintenance). Waiting until the end, of course, makes it easier to paint the model !
However, the same isn’t always true with regard to installing the motors. Issues such as drive chains and motor mounts need to be thought through, and may be better fitted as part of the main hull assembly.
It depends on the model, of course ! I haven’t seen the new Panther (yet), but installing the motors early is certainly, in my experience, essential for the WW1 Mark IV, and I’m still thinking it through for the M3 Lee / Grant.
Hope this helps,
Chris
PS. You also need to think about the type of batteries you’re going to use, where they’re going to go, how to fix them down so that they don’t move around while running, and how you’re going to easily recharge them.
Mark IV (Liesel, Abteilung 14, France 1918)
M3 Lee (25 Dragoons, Burma 1944)
Universal Carrier (2/Wiltshires, Italy 1944)
Panther (Deserter, 145 RAC, Italy 1944)
Centurion Mk 3 (8KRIH, Korea 1950/51)
Morris Quad, 25-pdr & limber (45RA, Korea 1951)