Hi Micah. Yes, there are three levels. The basic level (level 1) is a regular sandtrooper. You have a lot of freedom with how you put it together - check out the CRL on the 501st page. Sign up at the 501st - http://www.501st.com/members/join_form.php . Once it is built, send pics to your garrison GML. Your GML will contact 501st and approve your costume (or they will get back to you with required improvements/adjustments). After that, you can apply here at MEPD for PO (level 2) and then SWAT (level 3). Of course, the best way to do it is to document your build here at MEPD - you will get feedback and help with your build so you won't need to do any improvements at approval time.
For level one, a few things are optional (like a backpack) but keep in mind what you will eventually need if you are planning on going for SWAT (that is the level where emulating a specific trooper is applicable).
Hope that helps a bit.
https://databank.501st.com/databank/Costuming:TD_sandtrooper