Welcome to RIBnet, an t-Oileanach.
I'm a bit further down the county but know your stretch of water well. That's a nice enough combo - can't help with the price. The Etec 50 should be an economical engine. You'll find the special 2 stroke oil (XD100) can be bought cheaper on the internet or you can have it set to run on the cheaper standard stuff.
I'd have any secondhand motor serviced before use, to check it out. I'd be wary of anyone telling you they've had it serviced - ask for an invoice if they do
As regards the shopping list, I'd be with those who recommend a wee spot of training. You could do a lot worse than give Donald Cullion at Mevagh Diving
a call - he'd sort you out. That's a sporty enough coastline you have up there - it warrants a bit of preparation!
Two Stroke Mick (on here) uses a crowd in Sion Mills for his Etec servicing. I'd stay clear of Motor Marine