150 Rev 15mm works well on a Heckler.
That’s what i would go for. I also have a 1 1/8″ heckler from 2003, used to have a QR fox 32 Talas on it and then i got a 150mm Revelation RLT Ti which has a 20mm maxle and the difference is great, it’s the perfect amount of travel for me and loads plusher and stiffer than the fox 32 ever was. It just ploughs through stuff the talas would wince at.
The problem is you’re limited with the 1 1/8″ steerer so you can’t access the new generation of forks but a 150mm Rev would be perfect unless you want another 10mm travel but then you step up to something like an older fox 36 and you take a weight penalty.
Also, i would recommend not going for anything with adjustable travel like a Talas or a dual air rock shox as you take a compromise in the damping. So stay away from 2-step, dual air or talas.