they are a good motor at the price, well you get what you pay for but value for money they are up there, only the best motors that are much more expensive will be better so yeah go the trackstar.
i wouldn't go any less then 5.5t for modified i prefer 4.5t for mod and i have tried 5.5 t and 4.0 t as well 4.0 t was too much ( for me and the electronics )
get either the 4.5 t or 5.5 t don't worry about the track being small just gear it accordingly starting around 9 to 1 roll out for 4.5 or 8.5-8 to 1 for 5.5 , these are just suggested starting points but have that temp gun at the ready to test for motor heat. the 5.5t turn will be a little more forgiving with the gearing and if your new to it this is probably the best bet as you can always get the 4.5t down the track.
cheers