There is more than one 103 Stage IV kit in the catalog, and the pricing of course varies. If you're talking about the TC96/103 Conversion Stage IV kit, it comes with heads, 58mm throttle body, 259e cams, 10.5:1 pistons, SEPST, etc., and the kit alone retails for around $2250 before labor. There is another kit for a lower price that claims to be legal. It's cheaper because it doesn't include the SEPST but uses a Harley street legal download instead. I think that one runs slightly less than $1900. Do you already have an SEPST? If so, you don't need the kit that includes one.
I can't offer any answer to your reliability question, but before you pull the trigger you may want to verify exactly which 103 Stage IV kit this dealership is trying to sell. And does the $1200 price they quoted include just the kit, or is all labor included as well? If it's an all-inclusive price it sounds too good to be true for a Harley shop.
Jerry