3 98 ups

Well, what don't you like about the gun to start with? The stock barrel on a 98 isn't exactly great. The most common one I hear people mention is a J&J ceramic for a replacement.

I personally replaced my trigger first. The double trigger elminated a lot of cramping that I would get due to having a hand injury. However if your comfortable enough to play with the single trigger then you can wait for that.

The expansion chamber isn't really a must-have right off the bat. A 98 is, dare I say designed, to ignore the occasional burst of liquid CO2, which shouldn't happen too often, especially if your gun is stock otherwise. I have an electric trigger, can fire out 15 BPS, and still haven't had it lock up on me more then twice. Spikes with CO2 are however pretty common. If you switch to HPA then the X-chamber just becomes useless.

So the generic advice would be the barrel, the personal advice the trigger. I doubt anyone would say that the X-chamber should be the first thing you look into.