One thing to bear in mind with the 9.9/15 choice is in most makes they are the same outboard just with some different parts/tuning. In my book there is no point carrying the weight of a 15 and only having 9.9hp... so I'd always look for a 15.
One problem, as you may have realised, is that the 15 is the most desirable 2-stroke size because of their popularity for SIBs. Nice condition ones of the latter years (2001-2006) fetch £1000-£1300 in the peak season and just a little less this time of year. £750 is a reasonable budget for a decent one now we are getting to be out of season if you wait and get lucky. £300 is the price for spares or repairs motors and £500 often only gets older motors I'd not be keen relying on at sea.
Choose wisely and whatever the budget only buy something that is demo'd and starts/runs/idles/pumps water perfectly.
If you only run 2 up then do consider an 8hp because that will get you a far far better outboard for the money.