Ok, first question. Zero. There are no real upgrades to the Excalibur. If you want a different barrel it takes autococker barrels. The bolt is a lightning bolt and the valve is a tornado valve. There is talk about an LCD aftermarket board on this fourm but it is still in the prototype phase. They do make a highrise for the excal but the choice of highrise vs. lowrise is personal so it doesn't count as an upgrade.
Second question, that depends on what happen to the solenoid. If you went and disassembled it and couldn't get together then you would have to buy a new one. If it broke then it is free. The last I heard there were no solenoid upgrades that could be purchased.
Third question, it comes with the Sidewinder regulator. By adjusting the pressure on the sidewinder, you adjust the velocity of the paint.
If you are looking at an older gun you are going to need to know two things. First, what generation it is, and second, if it is an older generation then have the factory upgrades been done to it. If they haven't then you will want to have it sent to AKA to have them done. These are free upgrades that AKA will do to the Excalibur for there owners. Since you are buying it used you will want to contact AKA first to make sure that the warranty will transfer over when you buy it. If it is an older Excal then your going to want to know if it has the generation 3 board in it. That is what you are looking for. There is nothing much that can be done if it is a generation 1 or 2 body, which I think are the same if I remember right. And there is nothing wrong with the older bodies, you just want to make sure that it has the gen. 3 board.
"Whom shall I send? And who will go for us? And I said 'Here I am, send me.'" Isaiah 6:8