I have has a Wanhao 4DS for about 6 years now and it is still performing OK despite a lot of use. The most expensive replacement was the dual extruder head, but the old one was probably OK but needed cleaning and setting up, but I just lost patience with it and then found the carriage was distorted and that was the root cause of the problems. Now it is printing well with just a bit of shimming to correct the misalignment.

I bought a £70 Anet I8 kit before Christmas and it has become a project to beef up the plywood frame, make it electrically safe and put in better slide bearings. I haven't tested it yet, but it will be interesting to see how it performs against the 4DS, which was nearer £700.

The only other upgrade to the 4DS is a WIFI SD card, which is beautiful if the printer is less than 3 feet away from the WIFI dongle, but over 5 feet and it doesn't connect at all.