Hi Craigu,
yes, you can use any public launching ramp on the river (although some are a bit shallow IMHO). A boat with a swing keel is a necessity for trailering. You COULD trailer a fixed keel boat, but you'll soon get sick of that. The ramps inside yacht clubs are for members only.
I started out my sailing journey just like you - lived up in Duncraig and got hold of a dinghy to sail in the river. The dinghy got very small very quickly, so I upgraded to a Hartley TS16. I used to launch at Matilda Bay ramp and pull up on the beach next to it before setting out.
I didn't have an outboard, but as long as there is wind you can sail. It's when the wind dies and you find yourself 2km from the ramp that you will wish for one. But a small boat only needs a small motor.
The Hartley would be a great boat - nice and stable, easy to launch, spacious and much drier than a dinghy. There's one on facebook at the moment:
www.facebook.com/marketplace/item/906557633316305/I haven't seen the boat, but one thing to be aware of with all Hartleys - they are old boats made of ply over timber frames. Check carefully for rot and delamination of the timber. Also, be aware that the ply is not as strong as fibreglass, so will break on impacts where fibreglass would bounce (ask me how I know).