A custom tune is the best option – so yes, a custom tuned Van R (with or without Push) would be top dog for £250 – with hopefully the correct spring and tune for your riding once you’ve spoken to the chosen tuner. My understanding of the Push tune is that it is a worthwhile upgrade as it gives more internal oil flow which helps with small bump sensitivity and recovery from large bumps.
A stock shock on the other hand has to fit many bikes and suit many people, so other options like bargain Swingers or Rocco’s would not be custom tuned and hence almoot cetainly not as good. They may have more adjustments… but if the custom tune is correct you will not need to adjust anything.
Fox shocks are the standard – and there are so many tuning options which are well understood. Getting the same level of support for other shocks might not be so easy – Rocco’s are well known for problems, and I’ve heard older SPV Swingers can be tuned once the internals have been swapped out for shims… I would (and have) stuck with Fox because the chassis is good, and you can get many things done to the insides.
Diverse Dueler looks like excellent value with the Ti spring – if you can stretch the budget.