Holy hell there are 30 Synology models. Uhhh.....crap. I see people saying "5-bay" and "4-bay" Synology.....which one in particular are you dudes buying? I've spent an hour on this website trying to compare and contrast various models, but since I have 0 experience with NAS I have no idea how much RAM, or CPU I need (it looks like these are the determining factors of the models). I assume I need 4 Bays minimum (I'm shooting for an 8TB Raid 5, so 3x4TB drives), but that's about all I know.
As for what I plan on using the thing for (as I'm betting this will help with any sort of recommendation), it's only going to be a video, picture, and music storage machine that is accessed rather infrequently. I'm not trying to create a work server here!
EDIT: Now I'm debating how badly I even need a 4-bay and RAID 5. Realistically, my pitiful little home network probably only needs a RAID 1 config (2-bays) and I see this saves me money. Now I'm thinking that's the way to go. Ugh.
Here's something I learned early on about Synology. The model number has a lot of info in it. The first part of the number tells you the maximum number of drives it can take and the second number is the year it's released. I have a DS1813 which means it can take up to 18 drives and was released in 2013. The unit itself can only take 8 drives, but there are two expansion bays that hold 5 drives each which will take the other 10 drives if I expand. Not all of the Synology servers can take the expansion bay so this is how you know. If you look at a DS415, that means it can take 4 drives and was released this year.
So how do you decide? Well first off it depends on how important the data is to you. At a minimum you're going to have one drive for redundancy. So in a 4 drive enclosure, that's really only 3 drives of space. In a 2 drive enclosure, that's only one drive of space. In the event of a drive failure, you can only have one drive fail before you're vulnerable. I prefer to have two drive redundancy because it can take awhile between the time that the drive actually fails, you realize it has failed, getting a new drive, and then the extensive long process of it rebuilding the data on to the new drive. At any point in time if another drive fails. you'll have lost data. So that's why I prefer to have two drive redundancy since I'd have to have three drives fail before I'd lose data and I have the other drive as insurance while it's rebuilding. Granted, you should also back all of this to the cloud like Crashplan for offsite storage, but I still like the comfort of having two drives fail without losing data.
Don't worry about the CPU, or anything like that. Unless you're transcoding video on the fly or going to use it more as a server than storage, you're not going to really need the performance. Concentrate more on how many drives you think you'll need. Also, whatever you think is the right spot for the number of drives, increase it. You'll realize you wish you had more later on and it'll be harder to upgrade then. Pay up front and give yourself flexibility to grow in the future.
In my setup, even though I can max out at 18 drives, I'm right now only at 6 with two drives for redundancy, which means I'm really only at 4 drives of usable space. That gives me plenty of room to grow. I don't think you need 18 drives, but I'd certainly would go more than two, and I'd even say more than 4. I'd look to see if the DS1515 is affordable to you. That's a 5 bay unit, with the ability to expand with two expansion bays added to it. The 5 bay with two drive redundancy, will give you 3 drives of usable space. If you fill them all with 6TB drives, that's 18TB of space before you need to expand.