I have pretty limited boating experience -having not really grown up boating I have only the experience of the boats I have owned. 1st was a 10ft/10hp tinnie then traded up to a 12ft/18hp and now have a 14ft/40hp. I would recommend a boat that you feel comfortable in on the ROUGHER days rather than banking on the weather being perfect. There’s plenty of great skippers on this site who can probably man a 10 or 12ft tinnie at the end of the walls on a rough day, but unless you are great at the helm in the rough while casting/fighting a fish then its something to consider! I know personally I wouldn’t feel comfortable in much less than my 14ft plate tinnie – but its personal preference.
Like Craig said, I would say get out there and do a few trips in a variety of weather (that you’re comfortable with) and go from there. Maybe the few grand on a new motor might go towards a bigger boat…any excuse hey! Tim