It will be a 757. Don't know if you can pre-book seats as it is a charter - I guess your travel agent will have the answer to that. I'm sure you will be able to request seats at check - in and there may be a charge for legroom seats.
I think most of the time the loads are around the 100 mark and on a 213 seat aircraft - that means pleanty of room.
Hope this helps - and is an acurate answer (perhaps someone else might care to add to it).
Regards,
BP