For the guns discussed here I own an AC556K, an FNC (had two of them at one point), and a couple lighting links as well.
If I was to put a value on them the RLL are the best option of the three in my estimation. They really give you full M16 capability with the sole exception of 22LR.
The FNC is a good shooter and well looks like a machinegun. Its real hang up are spare parts. I don't know if there will ever be a plethora of spare parts available or not but assuming import/export restrictions continue to get tighter the odds of FNC parts kits coming in shrinks in my estimation.
That said if you are only going to shoot the FNC a couple time a year and not treat it like you stole it, than it should provide many years of faithful service. Granted anything can happen at any time, but odds are it won't break or wear out anything in the first 10,000 rounds assuming the prior owner didn't beat on it too bad. That might be a decade or more of shooting for you.
The AC556 isn't as good of a shooter as the FNC and of course looks like a Mini-14 (albeit I have personally always liked the looks of the 13" folder K variant). The good news for the AC is most wear items are Mini-14 compatible. Springs, barrels, bolts, extractors, pins, etc. In general the only parts that are not Mini-14 compatible are the select-fire specific fire control parts. There is really only one part on the AC that is easily damaged or lost and that is the selector actuator arm. There are really no shortage of AC556 parts kits out there as Ruger made post samples for a decade or more after the 86 ban and they pop up with pretty decent regularity when a PD demills their ACs in trade.
In regards to your question about the 3rd burst parts. Original FN sourced burst parts are really rare. There are also some Pindad burst kits out there as well. However, Todd at TBA Suppressors makes domestic FNC burst kits that are a clone of the original FN produced parts. One of my FNCs had an FN burst kit and I had a TBA for the other one. I couldn't really tell the difference between the two kits and the TBA burst kit appeared to be very well made.