I think the issue now is that HL had recoil on their IR tanks, but they are now, according to rumours, dropping their non-standard IR and only doing BB's. The BB units didn't have recoil, though some newer units do have a recoiling BB gun from what I have seen.
The HL recoil units can be bought for under £10 on Ebay.
If you want recoil with an RX18 the only way to get it would be with a recoil unit, but that may mean no BB gun.
If you want a recoil with an aftermarket board then you can make your own mount or chop up an HL recoil unit.
Clark also does a one size fits all servo recoil unit, which some may prefer as it is an easy route.
Tamiya and the old SLU units use a mechanical recoil. Tamiya used their own, SLU configured theirs for Tamiya or Asiatam recoil units.
SLU, the new ones, support servo recoil.
Elmod use servo's, or mechanical recoils etc.
The short answer is that because HL use a customised split channel signal system, their system can't operate a servo, it has to be a mechanical one, which is why they have designed their recoil the way that they have.
Mechanical recoils tend to be very noisy and unrealistic, whereas a servo is silky smooth and adjustable.
