Main ones are the size and weight of the finished model...a maximum length of 40", and as light as possible.
Preferably an abs/styrene boat, although fibreglass would be ok, I'm pretty sure I can bribe my old mate Andy to do any hole cutting for me if it's needed.
Preferably a single motor/single prop boat...this is due to financial restrictions at the moment.
An X craft, Type XX1, a boomer...something like that.
Have a look round in the back of your sheds and workshops and see if you have anything you'd be willing to sell me.