he t30 has 8gb of ecc ram should i choose the 8gb for the vm
I think you are going to have a little trouble here. FreeNAS requires 8GB RAM minimum to run stable and you are planning to run an OS and then a VM on top of that OS, and you only have 8GB RAM. You will not have 8GB RAM to actually give FreeNAS in this situation.
lso the cpu is a e3-1225 xeon with 4 cores. Should i choose 4 cores for the vm cpu?
As for the rest of your questions, FreeNAS works best with 2 or more cores however more doesn't always help so I'd give it two cores, that is all I give mine.
And lastly if something were to happen would i then not be able to recover the data on the 3tb drives?
It depends on what that something was. If someone burned down the house then the data may not be recoverable. If you accidentally deleted your FreeNAS VM and didn't delete the vmdk drives then you could recover the data. I don't want to tell you that everything will be okay, FreeNAS is not fool proof and as with any computer device, many things can and will go wrong. FreeNAS is not a build and forget type of machine, you need to set it up properly so you get email notifications when something goes wrong, it needs to be powered by an UPS that is properly configured, and if it works then you don't upgrade the software just because there is an upgrade. Also FreeNAS was designed to run 100% of the time, not be powered up and down all the time. And you as the user would need to read the user guide cover to cover and need to educate yourself with ZFS. FreeNAS was never designed for the novice person but we are seeing a lot of novice people come in and have a terrible time. When I started I was very familiar with computer systems although FreeBSD and ZFS was new to me. I did a lot of reading before I purchased my first system, which by the way everyone on this forum said I was buying hardware that would not work. It's a good thing it did work. I did eat some crow about the NIC not being very good but I think that is what made me credible here. I tested the crap out of the NIC and a new NIC and posted all my testing results, it was a long one.
So my point is, ensure you learn about FreeNAS and how it works and understand how to replace a failing or failed hard drive, don't show up and ask how to replace a hard drive. FreeNAS is absolutely the best Bang for the Buck and has a very high throughput compared to even more expensive qnap or synology equipment but it's not a leave it alone system. If you do leave it alone then you will be another person here that we see who says "I built the system 2 years ago and now I have a problem, please help" and the problem is typically a hard drive failure or a corrupt boot device and they never kept a backup copy of the configuration file. Sorry if I am getting a little preachy, I've just seen a lot of these types of problems over the past month or two.
So my final advice is if you want to run FreeNAS on your T30, just run FreeNAS on it and use your 3TB hard drives as normal hard drives in the system, create a Mirror for your data and you will have about 2.7TB of redundant storage, oops, minus 20% for that to ensure a fast and healthy pool. Should anything happen to your computer you could take those two hard drives to any other computer and boot up FreeNAS and access your data.
I hope I didn't discourage you, I just want to be honest up front.
