Hi Jon, I guess everyone's been kind of dancing around the original question so I'll give it a go.
I have no idea about AMS, so I'll leave that to others.
I'm guessing as you have an IOR half tonner it won't be a one design class, so we can dismiss that option.

For IRC you will need a full hull shape measurement including weighing as a stripped boat (empty tanks, no sails, dry inside, nothing movable onboard).
There are 2 different ORC's. Both can race together, but ORCi boat for boat will have a better handicap, as more data points are measured and better data is generated.
ORCi (for ORC International) will also require a full hull measurement and weighing.
ORCc (for ORC club) is more up your alley as it uses generic data for many of the data points. It's a lot less measurement needed. First is to use the knowledge base here on Seabreeze to locate another half tonner with a similar hull shape that has an ORC certificate. You can use this hull file on your certificate. I think you still need to be weighed and maybe the overhangs & freeboards measured, as well as the other normal simple measures. Sails will also need measuring but you don't have to go through the REALLY hard measurements like inclining etc.
ORCc is not overly onerous, but obviously it's more work than just getting a PHS handicap issued by the club. A lot will depend on how much you want to get into it.
and that's my 2c worth...