The macro line between the ASA (Air system adaptor) at the trigger grip frame and the regulator is attached by push fit connectors.
If it is in decent condition and the integrity of the ends are straight then that should not have any leaks.
As its been off and in the garage do check it has been blown through before airing up as any dust etc will be blown to the filter before the regulator.
There could be other issues when you first air up, so a basic strip & service would be a good idea. But if you are not familiar then it may be a better idea to test before taking apart so you know what does or does not work before you blame yourself for not rebuilding properly.
For a barrel it takes Smart Parts threaded barrels, and in the UK they are still known as Smart Parts, but in the rest of the world they are now Gog.
http://www.smartpartspaintball.co.uk/
Manuals are available here:
http://www.smartpartspaintball.co.uk/support/smart-parts-support.asp
As everyone else has covered the Ion can be used on Co2 but there are potential issues. If using co2 then you ought to have either a cylinder with anti-syphon correctly installed for the marker, or using a remote line.
Its much simpler and easier to use air (still pretty cheap for a 3000psi aluminium/steel cylinder in the region of £30)
CO2 should realy only be considered if it is the only option at the site you will play