RR12, the Merlin is a fantastic way to start your sportive journey. It's an "honest" sportive in that the various routes cater for all levels of fitness. Don't be tempted to try the "long" route yet though, it is serious! The climb up the Black Mountain is very satisfying when you get to the top, and then the descent is brilliant!
As Lukesdad has said there are a number of clubs in the area, Bynea, being probably the most famous. However, they are nearly completely TT orientated as far as I have been led to believe.
As for bikes, as you're in the Llanelli area I'm presumong you've been to see Peter in county cycles in Cross Hands? If not, I would. Great shop, fantastic advice and service too. And dealers for Wilier, Ridley and Trek.
Hope this helps?