One other thing about the WELL is that since it has a smaller cylinder it is harder to get high fps and it is also not as good at pushing heavier bb's because of the smaller cylinder. But yes, there are parts out there for the WELL VSR10 but from what I have heard, the quality on them is not as good as the main stream brands such as PDI, Laylax and Action Army.
Regarding what parts you upgrade first is your choice. I tend to upgrade the trigger first as I always put in a bigger spring. I find that if you have high fps, the right length inner barrel and the right weight bb, you can get really good range and accuracy. Even if you use a stock 6.08mm inner barrel that is 430mm long and a m170 spring with 0.4gm bb's, you should be able to reach out and touch people at 75 meters often. Also, one really good thing about having a high fps rifle with heavy bb's is that over long ranges, the wind does not push the bb off course so much. So you could look at it that fps creates accuracy too.
So I guess it depends, you can get accuracy from upgrading the barrel/hopup group first but you can also do it by having a bigger spring, trigger and heavy bb.
The barrel group is really important though. If you do not upgrade it and you find the stock hopup chamber and inner barrel to work well, then I would suggest at least doing the Top Dead Centre mod so the bb can travel straighter.