TravelBug82,
Welcome onboard
I'll have to check the aircraft type in the morning for you as I'm mobile so don't have it to hand. Whether to pre-book or select seats is up to you really. Seating is assigned using a best fit model taking in to account pre-book and selected seats so remaining passengers are slotted in the best they can. The majority of passengers under the same booking will be seated together. However, the nature of aircraft seating means it's inevitable not everyone will be seated together. For peace of mind or if you want to sit in a particular part of the aircraft, you may wish to pay the extra.
EDIT: MAN-RAK-MAN (TOM312/3) is looking like a Boeing 737-800.
Darren