I've never had the funds to just buy a full kit, so the option of slowly upgrading has been my preferred method. And since I don't see any really advantages to a coil over conversion, it is a win-win in my book. Not sure how it could be more expensive looking at the list Blu posted, but even if it is at least I can spread the costs over time and end up in pretty much the same place.
Something for the OP to remember is that that (as far as I know) looks like HDK is the only one of the aftermarket kits that include options for slant 6 motor mounts. At least I don't think so, I only looked back to see if Denny said so so maybe the Alter-K setup does as well? If not, you will need to be ready to "roll your own". Something to think about.
Last comment on the aftermarket kits - I'm still not comfortable with the lower control arm mount. If I were to make the switch, I would look at a HDK since (IMO) he takes into account the how the coil over loads the chassis but it still has a single mount resisting any rearward loads the lower control arm receives from pot holes or anything else the tire contacts. The OEM MII suspension had a strut to resist those loads with a narrow LCA. The aftermarket kits I have seen replace that setup with a wider LCA and a bigger mount (pretty sure they used to be pretty flimsy), but it still feels like it is at a disadvantage. Might be that they have run the numbers and it isn't any less stiff than the OEM setup was, but it makes me wonder.