Alright! Answering my own question here.
So, I went with the TS-453D (8GB RAM) and I generally setup CrashPlan following the instructions in this video which were indeed helpful.
The trick was, selecting the correct folders for backup in CrashPlan. The video makes it seem you can just easily select the folders you would like to backup on the NAS -- not true! I had to drill down to /share2/CACHEDEV1_DATA/_the_folder_I_shared -- when doing that, CrashPlan appears to be working well.