1. So without knowing where you are going, how can you do a dive plan?
2. How late can you wait and do your dive plan?
3. Will (and how soon) will the DM give us all the necessary varibles, e.g. tell you the depth of the wreck and when to come up etc.
One quick question...where is PCB?
Your dive plan...creating a dive plan has been done pretty well at the last minute. You may learn details at the last minute that will impact your dive plan. For instance you may know you're going to wreck 'x', put until the DM ties off to wreck 'x' you will not know the vis, currents and where the boat is tied off to wreck 'x'. The DM will then tell you, we're tied off to the bow of wreck 'x' at 90fsw, the vis is approx. 60ft, there is a mild current of 1 knot, and current is moving from the stern to the bow.
There are other variables that you are in control of. For instance, when to ascend. This is dependent upon you and your buddy. If you are both relatively new and your on AL80's and the wreck lies in 90fsw, air may be your limiting factor. You and your buddy can determine this prior to your dive. You can say whoever gets to 1000psi first or if we hit our NDL's first of the most conservative computer, could easily be pre-establised as part of your dive plan. The DM may only say to not exceed the limits of recreational diving or make sure you get back on the boat with 500psi.
You can do some digging to find out what typical profiles in PCB are and make sure your gear is ready for diving in PCB. If divers typically carry SMB with reels and pony bottles, you may want to make sure you have SMB with reel and pony bottles.
If you're concerned about NITROX, you could simply go with 32% which will give you a MOD of 109fsw at 1.4 ppO2. A charter that caters to recreational divers isn't going to be going for real deep dives. 32% will cover a lot.
Finally if you are that uncomfortable about planning the dive, ask the charter if a DM will be diving with you or see if you can't hire a DM for the dive until you get more comfortable with planning and execution of dives.
A little tidbit. If you're diving wrecks (again, where is PCB?) you can start off deeper on the wreck and then imagine diving upward spirals around the wreck. Hopefully your using a computer. This is multilevel diving and air will be your limiting factor. I've had dives where I've maintained 1 minute of NDL for most of the dive!