I wrangle almost daily with the inner voice that encourages me to upgrade and have considered a 105 groupset. It's the big kid in me that likes new toys delivered in the post.
But, I have resisted (so far) because:
1) In truth whilst it has its detractors, the kit on my bike (Defy 3.5) works plenty well enough. Yes there is better out there, but will it so greatly enhance my ride enjoyment/ability? Unlikely. Or at least not relative to the cost. I always have a blast when I go out and the bike does everything I ask of it. In fact, its me that needs an upgrade as I can't always do what i ask of myself when i cycle, but that will come with time.
2) I know that cheaper upgrades will likely reap greater improvements. Notably tyres and wheels. I'll be seeing to these as a priority when I simply can't resist spending some cash any longer.
It sounds like a second hand bike from a big brand manufacturer could be a good starting point for you. As long as you look after it, it will hold a reasonable value should you decide to sell it on and buy something kitted out with higher-spec components at a later date.
Just my couple of pennies worth.