I would say get a barrel kit when you can because it is worth it's weight in gold IMHO.
This will allow you to taylor the barrel to different sizes of paint, which varies more than you think from brand to brand even batch to batch. That being said buying decent paint will have a big bearing on overall performance.
Most decent markers use A/C (AutoCocker) thread these days (Eclipse, Dye, Empire, Macdev, Bob Long) which enables you can continue to use you barrel kit when you upgrade you marker. At worst if you new marker did not accept an A/C threaded barrel, with the freak all you need do is buy a new back. There are also wide range of different barrel fronts available too which will enable you to customise the overall barrel length as well!
Pre owned kits come up occasionally and can be had for as little as £80. That will include many if not all of the inserts you will ever need. I have been using a Freak kit for years and years and still go back to it as my go to barrel system despite owning many others, including hammerhead Dye, Assassin, and WGP et al.
Deadly wind carbon barrels are also worth alook they can be had for around £80 new and accept freak inserts, a double win!!