Should be pretty straightforeward, I was tinkering last night and thered a definate hollow in the seatpost that lpoks like it’s designed to give clearance to a coil, and ther are plenty with coil shocks about.
As for which one…..
£270 would get you the Van RC which is basicaly a DHX shock missing a couple of adjusters, but if you shop arround the cane creek isn’t much more (£380 from jensenUSA) .
S/h you’d have to find a bargain, if your upgrading to coil presumably its for performance, so you’ll want it regulalry serviced and tuned to your weight, so add £80-£150 to the purchase price (service + tuneing + reducers + new spring) and your pretty much in new shock teritory.