Add me to the list saying don't put big bucks into incremental upgrades. If everything is basically OK, wait until you can afford to replace the bike with something better, and keep this as a "B" bike, for bad weather training, commuting or simply as a spare.

If there are one or two items that need replacing, such as only the RD or shift levers, you can do so as needed. I'm fairly sure (but don't swear to it) that all Shimano road components are compatible with each other (with the same number of gears), so you can replace individual parts as you need them without replacing all at once.