The D2824 chassis is single conversion. The "single conversion" and "double conversion" applies only to AM mode. The dual conversion chassis is a little better for AM RX.
Most people prefer the MB8719 boards because they are more "extra channel" friendly. You can change the programming on the PLL directly thru the pins.
On the D2824 PLL, it's done in BCD ROM, and can't really be modified, however, they have a reference divider circuit out now (look up Galaxy "N" kit) that can allow channels thru the reference output pin of the PLL.
It's my personal taste, but I prefer the D2824 chassis, the NB/ANL circuit seems to be a little better designed, the AGC is nice and tight, responds well, and rock solid on TX frequency. But that's just my .02 worth.
The TRC-450 has a mic gain on it if memory serves me right, whereas the TRC-451 does not.
~Cheers~