Hi thewre and welcome to the community, I am positive you will fit in well and besides, you have to start somewhere.
First things first, yes ANY 1200 can take a hard drive so long as it's 4GB (limited by the 3.0 ROM's or OS), you can get bigger but it will only reconize just over 4GB.
This moves me swiftly onto the OS which can fix this issue by using 3.9 or even 4.0 onwards if you have a damn good PPC setup. Any OS will work in any form of case, it's about the hardware you use not the case setup.
Finally yes the trapdoor slot is for your accelerator and yes most (if not all) Accelerators offer (usually SIMM type) RAM slots so you can upgrade the RAM too at a fairly cheap price.
My question to you is what will you be using your Amiga for? If it's games and general use (no OTT programes) then I'd get yourself a good 68030 Accelerator like mine in the 1200T (in my signature). If you are just mostly into hardcore programes and want a very fast machine then id look into 68060's or even a PPC if you can afford such a thing (which sadly I can't

).
I hope this helps you and good luck for your future in Amiga computing!
