No need to get grey hair over it. That will come soon enough with or without the trailer. Trust me!
There IMHO are only few things you/they really need to know before they mount the hitch. What size is the ball you will need to match the trailer and the weight of the trailer? The answer to those two questions will have an impact on what class of hitch they put on your tow.
Once they decide what class of hitch you need then they will be able to tell you what the size of the receiver mount is on the hitch. Once you know that then you will have to make sure that any ball mount/receiver you purchase matches that size. They come in a couple of different sizes - depending on what class and make of hitch you get.
You will also have to note the size of the hole on the Ball Mount/Receiver you purchase to make sure the size of the bolt on the Ball you purchase fits in the hole.
Your idea to purchase a couple of different ball mounts/receivers is a good one - but wait until you know what size the hitch they are mounting takes. U-Haul
sells ball mounts and they are normally a bit cheaper than Canadian Tire or Walmart.
If you are going to error on the towing level IMHO I would error with the tongue of the trailer ending up slightly down rather than pointed up. I purchased my trailer in Idaho and only had one reciever with me - to the eye it looked pretty level and I towed it without a problem back to Vancouver. I later changed the ball mount to drop it an 1 1/2" and noted a better tow.
I towed my trailer all over the Island last summer on my own, including a couple of ferry rides. I have even towed it through downtown during rush hour more than once. You will be suprised at how fast you will become comfortable with towing. Just keep your distant from the car ahead until you get use to stopping and keep the speed down on the freeway and you will arrive safe and sound at the meet.
Good luck and look forward to seeing you at Langley. You are right there will be lots of folks there willing to give you there ideas on the best height.
There is so much help and answers to questions available here. It is too bad that i dont speak this language yet and dont have a clue what anyone is talking about. (well 1/2 the time anyway) I dont even know if the size of ball mount has anything to do with my trailer hitch. Do they have to match up? I was always assuming so. If so what now, go down and measure the hole? Geeesh. I sure hope the trailer hitch people can handle this b/c i think i will do my best to measure and hope for the best. I am really doubting whether i can get them the correct info.