The springs on my 109 project have well and truly rusted solid, and the dampers are equally suspect. So what's the best options regarding replacement? Conventional springs or parabolic? Which dampers? All too often any comparisons are between knackered originals and shiny new replacements which isn't very helpful.
Not expecting the truck to be heavily laden with bricks or do heavy towing with it, so something softer on the rear might be preferable. Though I might yet put a soft-top on it with a couple of benches in the back. Is an anti-roll bar available and worthwhile on leaf springs?
Do you have a budget in mind?
Sadly I don't think there is a whole heap of choice on the market today. Parabolic springs have the potential to ride much better. Although many (at least for 88's are heavy duty so still ride very hard unless loaded. So check out the spring rates).
The stock springs will soften up with use, but this may take quite a bit of time and require you to load it heavily. And may depend on what springs you have on there now. As there were different rated spring packs back in the day. And of course you have no way of knowing if the springs on it now are OEM or have been replaced at some point previously.
Same is true with renovating them. No reason why you can't do this, you can even remove of add leaves too to alter the spring rate. It just requires time and effort to do. So is the best route if you want the cheapest. Putting new springs on will be a lot less effort.
Parabolics will induce more body lean as they are generally a more compliant spring. So the addition of an anti roll bar might be sensible. But probably best to try first and see if you feel like it needs one. I think there are kits for doing this, but will likely require a bit of R&D on your part.
As for shocks. This depends on the springs. The stock shocks or direct replacements will work fine with stock springs. With parabolics however they will get overworked and you'll want a more modern shock.
The shock length is also what largely dictates the suspension travel on a Series motor. So if you want it more off roady with more travel you may want to consider some longer shocks than stock ones. Depending on what you go for you'll be fine with the stock mounts, but may end up bending the spring backwards under full compression. Extended shock uppper mounts would solve this, but would have to be completely custom.
I've always like Heystee, but they are pricey and last time I look they didn't seem to have much for sale (suspect the market has shrunk as there less Series vehicles today than 10 years ago). They offered their own design parabolic with +2" shocks.
Rocky Mountain spares also do/did offer some nice parabolics and +2 ProComp shocks.
Lastly and no idea if possible. But the Santana PS-10 (looks like a 110) is based on a 109 platform. It used parabolic leaf suspension. Which I suspect but don't know for sure would fit a 109. If you could buy some OEM spec springs for a PS-10, it would probably suit a 109 very well, assuming they fitted.