This would be better off in the Mechanics forum, I think.
You want to stick with Shimano road hardware at 10-speed and below. This is what's compatible with your shifters. Tiagra is the highest still available, and par with what you have. The "trekking" rear derailleurs like the one you have are compatible but rare in the USA.
If you want to do a wholesale upgrade to 105 or Ultegra, look at Ribble Cycles in the UK. But I agree with @
pdlamb, you don't save money by spending it