Think you are gonna really enjoy this hobby!! So many facets...
A couple of which you are already battling!!
My 0.15 rmb is that you need a better idea of what you want to do before purchasing track! You might change your mind several times before deciding what you would really like!!
I'm not sure if you have spent much time on the "givens and druthers" yet. Maybe you should put a loop of track on a 4x8 and run a train, just to satisfy your sweet tooth!!!
Then decide if you will depict a real area, a specific era, if you want your layout to run continuously or do a lot of switching, etc., etc., etc. Think about "around the walls" vs tabletop! (like Andy said!!) Industries, cityscape, mountains, tunnels, water, all possibilities affecting you plan!
It just seems to me that buying a slew of code 100 track at this point is somewhat premature. but it's your railroad, so your rules!!