Not sure if new components is your best upgrade but I just took my LOOK KG281 frame out of mothballs and built it up.
Since I have 2 other bikes with Dura Ace components and I am going to use the LOOK sparingly, I decided to try the Tiagra groupset which is a step down from 105.
I paid under $300 for a complete groupset including chain. I put the components on myself and didn't have to tweak anything - it shifts perfectly.
If you don't mind having a 10 speed, I think the value can't be beat. I understand now why the big manufacturers are offering Tiagra equipped models.
Over the years I have ridden 105, Ultegra, and Dura Ace components. If I'm on a strict budget in the future, I wouldn't hesitate to go with Tiagra.