Lets say if i upgrade the ram , is lenovo model is stable enoigh compared to dell ? Ibalways thought dell is superior to lenovo but could not find proper comparision for these 2 models
They're both good companies. Check the depth of driver updates (i.e. who has the latest firmware and driver support). Are you using FreeNAS or TrueNAS? If so, check their Linux / bsd drivers. Should be fine for both. Note though, the
Dell uses SoDIMM (laptop memory) and maxes out at 16GB, which is not a great amount for any servery. You'll need more IMHO. The CPU is fine though.
As John said, the RAM (lack of) and the fact that it only has 1GbE NIC makes it a poor choice for this. I would suggest looking into Intel's DQ77KB - BOXDQ77KB (if you can get it cheap) or SuperMicro E200-8D. Failing that, a SFF OptiPlex will fit a low profile 1GB (or more) NIC card to allow for an WAN / LAN separation. Id recommend installing Windows and then HyperV a PFsense and FreeNAS install. Try getting at least 32GB of RAM and give 24GB of that to FreeNAS, 4GB to Pfsense and 4GB to the Windows Host. That'll allow for a secure NAS.