"What build should I use for ticks 1-50?"
Medium or Heavy Units
A lot of people will be using infantry the first 50 ticks, if you get ahead and build medium or heavy units you will have the upper hand in battles.
But infantry have bonus damage against against Medium units, so it wouldn't make sense to build "medium" units to anti inft? Also Heavy units don't get an advantage bonus damage against inft...
The only reason you would start with something other than inft is because most people in early ticks build concussive units, you should mention that since medium and heavy units don't really give you the upper hand in battles.
INFT IS THE BEST CHASSIS!!! AAAAGH!