There are a number of possible solutions to your problem. The first thing you need to consider is how many cloud instances you plan to run on your private cloud, and what software you plan to run on each. This is so that you can get an idea of the scale of the resources you will need to make the system work properly.
Running your own private cloud can be done with a single server, but there will be an overhead for the cloud software that you will need to administer the whole system. Ideally, you will need a few cores for this, plus 4-8GB or more of RAM to get something that will behave properly. To this, you will need to add the amount of RAM and the number of cores you will need to run for the maximum instances your are likely to run for all users of the system.
You will need a lot of hard disk space as well, and the hardware should be as fast as you can afford to buy. You will notice the longer latency you get from cheap drives, whereas good quality fast drives will enhance the performance of the whole system. You could run SSD drives to run the whole admin system, then hybrid or fast high capacity disks for the storage for users.
You will also need to consider whether you plan to run Windows server software or Linux. Windows server will cost you a lot of money, whereas Linux will be free. The same consideration applies to your software environment. Windows desktops will all cost money, whereas Linux desktops will be free. The same rule applies for the actual software you run on the systems.
You can now look at what you would want, and compare the cost of that against how much budget you actually have. If necessary, change your requirements or your expectations until you have a plan that will work.
I Assume you are looking to run a full private cloud setup, rather than a cheap file sharing setup, then I would recommend you use something like Eucalyptus, OpenStack or Apache CloudStack. Each has a free version you can use to create your own private cloud, using entirely your own physical resources. The performance will be entirely dependant on the quality of resources you have available.
To give you an idea, we ran a test based on 4 servers with 4 x 6 core opteron processors, 4 x 128GB of RAM, 4 x 2 x 2TB fast hard drives, and 4 x 2 port GB LAN connections. Performance for running instances was generally better than a standalone Xeon quad core (equivalent to an i7 quad core processor) desktop with 8GB of RAM and 500GB fast hard drive, with 1GB LAN.