If you change to the newer 105 group, you'll have to change most everything as new Shimano 105 is for 11 and 12 speed rears. You probably want to look at Sora if this is a road bike and you want to keep the 9 speed rear. However you do need to check the specifications to make sure it'll handle the tooth counts on your rear and front. Though I suspect you can probably get better range on the rear if you wished. But I have not looked to see.

Probably shifts every bit as well. Smoothness in the old 105 might just be because it's been worn for 20 some odd years.