get a barrel that will suit the majority of paints if you are on a budget
My website has a comparison chart for paint to barrel match
http://www.ukcougars.co.uk then go to the technical page from the link you will find there few other things for cockers there as well
if the cocker is not a vertical feed you can get a power feed from Armson
http://proteamsproducts.com so that can take care of that
a revolution hopper is a must (or any agitating hopper you feel happy with) most all the secondary regs do the same job mainly down to looks/price
Look at other peoples cockers and look at what they have on theirs and decide what YOU like.
In standard form the cocker is good the performance upgrades are not huge with aftermarket bits, ease of use and looks is their main selling point the external velocity adjustment kit (sidewinder or rex) works as well as the stock velocity adjustment but it is easier to use than the internal one ...costs money though all cocker upgrades are dependant on how deep your pockets are
drop forwards are not for everyone a member of my team likes his marker at arms length...you may be the same?
my own cocker was bought used, I never shot a ball through it I gave it straight to an Airsmith and said "do everything" looks like a peice of crap externally but all the internals are Evolution so it works very well but then I had the money to do it ....you may not