18x is a generic MOS indicator for those who have not been selected yet wish to join under the guise of going into the program. Just like when you join for standard infantry, you are given the 11x MOS instead of 11B, 11C.
As far as your questions and worry about buying things for selection...
You will not be going straight to selection. You need to be at least an E-4. You go to standard infantry OSUT, then jump school. You pass those and get sent to a holding/training unit. Once you reach E-4, you can then go to SFAS. You'll have plenty of time to hit the PX to get anything you'll "need" for selection.
IMHO the 18x route is overrated and unnecessary. Go 11B. Get OSUT and Jump school out of the way then serve in a line unit until you get your E-4. Do a deployment and get a taste for the Army. The deployment will give you much needed experience in real world combat situations as well as make you appreciate the "big boy Army" if you do indeed get sent to that side of the house. You'll have anywhere from 18 months to two years till you can go to SFAS. Stay squared away, practice your land nav and ruck yourself silly. I spent my time in the SOCOM world and enjoyed every bit of it compared to the regular line units. You still don't know if you will even like the military. Join first, get a taste of it, then decide if you want to go SF.
I hate war as only a soldier who has lived it can, only as one who has seen its brutality, its futility, its stupidity.
- Dwight D Eisenhower