MSRP on the MR556 is $3295.00, dealer wholesale (lowest price you will never see) is $2595.00 so IMHO $2800 is pretty darned good.
Something break on a new HK? I doubt it.. but yes, parts are available. As of 2-3 weeks ago, 30-rd mags are $19.95 from the HK webshop.
If you don't take the deal, let me know and I'll give them a call. Decent deal all-around.