The problems you will face are not obvious and immediate problems. For example, a year from now, a server might fail, and you find out that you can no longer obtain that specific model of motherboard or RAID card that was used in that server, so you have to replace the entire thing (purchase all new). Or, you find that a drive in your RAID array fails and you can no longer get a drive with the exact same model and disk geometry as the other disks in the RAID, so you are unable to rebuild the RAID and have to basically start from scratch.
The main reason why you should go with real server type components is the support lifetime. You can be fairly confident that you can get replacement parts be it CPU, memory, raid controller, motherboard, or disk drive, for at least 5 years after purchase. Otherwise you will find that when your components die, you will be completely unable to repair them due to lack of available parts.
Good luck to you, but I've been down this road before and if you cut corners in the beginning, it usually costs you more money to do it right later on.