If one part of your drive train is excessively worn, the rest probably is too. Yes, I know a mantra put out by bike shops to sell more stuff, but my Ultegra equipped bike -skipping gears a bit, so i replaced chain and cassette (chain had stretched and cassette teeth were looking decidely shark like on my more used rings). All shifted fine on the work stand. First day riding was dreadful, managed about a mile. Chain was slipping, skipping. It was the chainring teeth. Somewhat bizarrely it was cheaper to buy a new chainset than to replace the chainrings.
Your question is it better to fix (replace a few bits) or replace (new groupset) - if your chainrings are worn, and your shifters are beyond a good clean and a lube (to get working properly again) then go for the groupset.
Groupset releases: Shimano tend to revamp Dura Ace about every 2-3 years, usually for a new innovation rather than just looks. Once this new innovation has settled down they roll it out down the groupset orders, so usually Ultegra and 105 get the new innovations, whilst the other groupsets catch up with the previous innovations a bit later.
As for groupset pricing versus individual parts -many companies that have own brand ranges often have groupset deals much cheaper than stores that only sell branded bike ranges. These groupsets are OEM kit, no different to none OEM kit, just different pricing structure for the retailer. Try Ribble, Dolan etc
Post edited at 07:57