Just want to state, any helmet that meets SA2020 is not a "Walmart-Type" helmet. A $200 SA2020 helmet is built to the same crash specs as a $2000 SA2020 helmet. I won't say a $1K helmet isn't worth it, because it is, but not because of it's ability to protect your head in a crash.
NASA TT - "Both front side windows must otherwise be in the down position while on track." Most racing bodies require front windows down so you should be fine.
One thing I didn't see mentioned, the best helmet is one that fits YOU well. So if you have a local shop that stocks SA rated helmets make the effort to check them out in person.
Racequip, Conquer, G-Force, Zamp, and Pyrotect all make decent entry level options. All should protect you fine as...
For those with spacers, especially 15mm front, are you getting any spray on the side of your car? I loved the look on my previous car, but it only looked good until I hit a puddle or it rained and the side of my car became a mess...