I have also looked at this. Running a 24V alternator with step down creates a LOT of risk without a good amount of redundancy, as it introduces a single point of failure (the step down converter) for the complete system and they are not known to be the most reliable parts. Many of the Buck converters use high freq switching and at those current level could introduce a lot of noise, so that eliminates the low cost parts. Also, MANY options for 12V alternator. 24v, not so much. Good luck finding an affordable 100 A version that fits in your cowl. I did find a 12V, 90A alt in the S-10 form factor that would fit and suspect a 100A version is out there somewhere. If you are willing to cut and form the cowl, many options exist.
If you can squeeze in a 100A 12V alternator, it may be enough. Be advised though that the process of going from 12 -> 24 volts will only net about 75-80% efficiency, so factor that into your electrical planning. You can expect a 12V 100A alternator to produce 35-40A at 24V after conversion. I found that a mininun of 8K BTU is required (12K is better) and those draw around 40-45A @ 24V. Don't forget that the rest of your plane will require around 20A for the first 5 minutes after start to recharge and probably 12-15A after.
re-engineering the plane for 24V is no small undertaking, as many things are not available inexpensively. Try finding a boost pump that runs on 24V, and so on. Things like Pmags or surefly are likely out of the question. Where do you get 24V trim servos, etc.
Other big problem is the need for battery capacity. AC is most needed on the ground. That 100A alternator might only produce 50A or less at Idle. Therefore need a lot of battery capacity to insure you are not launching into the air with depleted batteries and suffer an Alt failure. Maybe ok in VFR with Mags. NOT OK in IFR or with electrical dependent engine. If you need 80A and are only producing 50A, that PC680 will be dead in a few minutes and will have no real indication that it has happened, as the alternator will not be producing 14V at loads beyond it's capacity. The 680 is marketed as an 18AH battery, BUT that is at a 2 amp discharge rate. You will be shocked when you study the discharge performance chart and see what happens if you are drawing 18 amps. Gets much worse if you are pulling 25A out of it.
Larry