Mate of mine used to use a powerlyte Scepter kit, apparently they are very good. Redz Barrel kits are also extremely good I have heard
I used to own a freak, excellent if you get them for a good price, easy to use just be careful not to dent the inserts. TBH I only ever owned 3 of the inserts, blue gold (small and middle sized) and pink (flippin huge). Never needed anything else.
Agree getting different backs is cheap you can use the same fronts & inserts for all markers.
I have also heard good things about the Evil pipe kit