I did Persel waaaaay back in 1999 on my way to becoming a Helicopter Crewman
You've got everything right so far - so thats a good start!! You should definately turn up in dress pants, shirt and tie.......... although after the introductions you'll probably be asked to remove the tie!! And yes, no jeans during the selection (After hours is fine so have some relaxed dress to wear in the down time). None of the tasks require any real exertion and from memory overalls are supplied if the conditions warrant it.
My advice for the course...........
be yourself. Don't try and guess as to what they assessors are looking for - they'll spot the fakers from a mile away!! Be positive, confident and outgoing in the exercises, but don't overdo it!! One of the guys from my selection missed out because he kept interrupting others and was a bit overbearing!! Instead, listen to the other candidates ideas and suggestions - and make your own comments and suggest improvements if appropriate. If you make a mistake and fail an exercise - DON'T FREAK OUT!! I misunderstood some of the details and probably looked a little naive during the group problem solving exercise. I also failed my leadership task and topped it off by wandering off topic during my topical speech!! You'll show more character by getting back into the conversation and remaining positive than sulking in a corner!!
Above all......... HAVE FUN!! Its a great experience and one that will probably stay with you for life!!
Best of luck!!