Off-the-shelf Employee Evaluation software abounds! Why re-invent the wheel in the first place ?
Many business management functions, in areas ranging from HR to CRM, to accounting, to strategic planning [hint intended ;-)] etc. are such generic functions that a whole industry of service companies, shrink-wrap software and also mature Open Source project serve them. Such applications and services are well on their way to becoming "commodities", like say email management software or Web Browsing tools.
The real difficulty is often in selecting the proper set of solutions in this crowded and dynamic marketplace, but unless your company is trying to produce yet another Employee Evaluation solution [at which case you'll need to plan for a project in the order of man years...], most any pick you make, in a rush (say in a day...), will likely be better, and more cost effective, than any app you put together in a couple of months.
I've seen cases when the sheer difficulty of reviewing the existing offerings prompted companies like yours to decide and roll-out their own, instead; "after all it couldn't be that hard, could it?"
... NOT!
Once you have selected such a solution, you can probably use their "default" configuration as-is, or for the least as the basis of your own configuration. Then with a few tweaks of said configuration you'll have a custom solution which fits your company "specifics" and culture, while leveraging the extensive expertise about Employee Performance Management which was encapsulated into these solutions during the last decade.
Ideally, your solution evaluation/selection phase should be longer than a day (but not necessarily full time!), so you get a better grasp of
- the TCO (Total Cost of Ownership) of such solutions
- the general functional and cultural fitness of the solutions (i.e. how these match your requirements and your general company's expectations)
- the level of effort to parametrize/customize the solution
- the maturity of the company and/or product (with a eye for the outlook, i.e. will it/"they" be there two years from now...)
and therefore choose something which will likely serve your company well for years to come.