If you want to use your existing 88/4500 tank get the vertical regulator model, and your tank will screw straight into it. You could get it with a bottomline CO2 maxflo, which will also accept your tank, but the bottomline maxflo is very bulky.
New Designz upgrades are generally the best available. A new bolt should be the very first upgrade, followed by a decent barrel, followed then by some form of trigger upgrade (the stock impulse trigger is a joke.) If you want one step better than a blade trigger, look at the HSW I-frame. The other must-have upgrade is a Tapeworm, or a VFF mod (if your body allows it.) This improves consistency and efficiency, and prevents bolt stick.
After that you get towards performance tuning -- a heavier hammer to compensate for the lighter bolt; the ND RIP valve to improve efficiency and consistency, the list goes on and on.