Prices don't sound outlandish, it's just an expensive conversion. My Self Tacker Conversion for my I-20 cost $400 alone for the Track and jib car & a Jibsheet. Add the cost of a jib in there. and it is about the same. Price out the parts of your Spin conversion. Spinnaker is the biggie, I'd buy a used one to learn with, you will thank me later. Save you some start up costs and an expensive lesson when you shred your kite on some fitting you forgot to tape. Poles are cheap if you look around. Think I would do one conversion at the time. Most jibs can be converted to self tackers just by changing the tack plate.
Go to someone's boat that's setup like you want and photograph the setup from every angle, include a tape measure in the picture, then take some more pictures and you Might have enough info to do it right.
Good luck and keep us informed.
CARY