I'm a fellow CDT Audio dealer and I don't think anything else needs to be said about what you get from upgrading from the CL line to ES Gold. As for the UpStage systems, it's a very simple way to raise the sound stage above the dash and it works extremely well. You don't need to add an amplifier and you don't need to remove your existing tweeters.
The difference between using an UpStage component system and one of the separate UpStage add-on packages is tuning. The "UpStage Ready" crossovers only have a 3-way tweeter level switch, while the UpStage modules have 30 possible settings via 3 switches.
I'd like to point out that a budget is merely a spending guideline. It implies "low cost/easily affordable" because of marketing and advertising.