How much storage would you recommend?

If you have internal drives, add some big SSDs. I do most of my work on an M2 Macbook Pro with 4TB of internal storage, but I'm working on a project right now that has about 35 TB of ProRez raw files on two 18TB external G-Raid drives. I also have a fist fill of Samsung T7 Portable SSD - 2 TB drives that I use for temporary media storage and system backups. 

Internally, AE does not need more than 40 or 50GB of Cache for Adobe Apps. Any larger than that seems to cause random cache problems with stuck frames and bit cache files take a long time to empty.

I have 140 apps installed on my MacBook Pro, and the apps take up about 250GB. Throw in another 50GB for app cache, and that leaves me plenty for the I gotta have access to these files all the time folders. Add Dropbox or Google Drive, and I'm good to go. I start cleaning house when I get less than 200GB of storage available on my boot drive.

Us SSDs internally if you can afford them, buy good cables for your external drives, and develop a standard file and folder naming structure to keep your files straight. I've been using the same naming conventions since I started filing project, shot, and reel numbers when I started making movies on film in 1970. It takes me about 5 minutes to figure out where anything I have worked on in the last 20 years is stored.

Is 6TB of storage enough?

Not meaning to be the weird old man, but my work computer still has barely 2 TB. If you clean house regularly by flushing caches etc. and offload your projects to backups when finished, you don't need more unless you really plan on rendering long form movies in 4k all the time or have to have years of back catalog projects at hand all the time. See? It's pretty much a first world problem and even if it wasn't, even the best hardware can't mitigate a bad workflow. Anyway, with prices dropping daily it's probably a moot point. Get whatever you can afford/ are willing to pay for within the budget. AE is probably going to be the least of your worries, anyway. Nanite caches in UE5, particle sim caches in Blender and other stuff will likely produce more data than your intended lo-fi use of AE from what it seems...

Mylenium