Great advice from NavyHoops and USMCDad. My experience is so dated that I offer it only for comparison...
I was Echo 5-81. Started in late May (as I remember it). I was commissioned and graduated in mid-May and stopped at home to drop stuff and went straight to Quantico. Don't think we had any USNA grads in our class - I think they followed us in July or August. I always assumed that they finished later or took leave after graduation.
Speaking of uniforms, we graduated in whites which are no longer used. In those days we had to buy whites, summer +winter weight alphas, dress blues and all the assorted gear. I think today's Marines have one single weight alpha uniform. We must have reported in in suits - don't remember. But we were prohibited from buying uniforms until we had the necessary instructions and classes. Like you guys noted, there were LOTS of uniform inspections by the end of TBS to make sure you had the whole seabag and that it fit properly. I want to say the entire uniform and complementary parts were $4-5,000 at the time (including the sword). Don't remember loans being available except via the Marine Shop or MCX directly. I know USAA offers a kind of starter loan to midshipmen and 2LT's today.
Our Birthday ball was definitely on base in Quantico. I believe we had a TBS only ball but foggy on that.
Agree with NavyHoops on buying field gear. No need to get too much until you get in the real world and learn what you need. In those days, you didn't want to be the guy with all the extracurricular "special" stuff. It set the wrong image and tone.
Thanks for sharing your experiences - they bring back great memories and provide a distraction from a long afternoon in the office!