The first thing you need to do is see if your old tank is still good. You can find the hydro date on the tank, and its usually good for 5 years. HPA is always better than Co2, but an A5 will be fine on C02...mine runs great with it...and I dont have an expansion chamber. For the price of an Xchamber you mihgt just want to grab a steel HPA tank.
Barrels. My take on it is just get a .685 or .689 barrel and be done with it. Paintballs are different from real bullets, a longer barrel does not help accuracy....it hurts it. Accuracy is determined by bore to ball match, so a 685-89 should work on everything. . An A5 is pretty long as it is, so you would want a 12-14 inch barrel max. I personally have a 12 inch barrel,. As far as barrel brands, Lapco($50), Custom Products, or J&J(both $30) are great. You really don't need more than that. In fact there are some great deals on A5 barrels right now(~$25)..
The only other suggestions I have is to make sure you have a good THERMAL mask. Dye, JT, Empire, Sly, V-Force are great masks. THe Thermal prevents fogging. I would suggest going to a paintball store and trying on different masks. Masks are not a one size fits all type of thing, you need to find the one that fits YOU. Finally a good pod pack helps in carrying extra ammo. You can get some great deals on them if you shop around($20 and under).
I shop at ansgear.com They have some great sales going on now. If you need any other help just IM me.
Oh yeah last thing invest in a cup. I've been playing since 91 and never wore one until I was double tapped there last year......yeah that hurt