Customized Estimate app similar to Zoho Invoice

I have an existing Zoho app to track clients and tasks that I would like to add an estimate module to. I like much of the functionality in Zoho Invoice, but I would want to tweak it, so just linking the two is not the answer. Also, once we have Estimate working, would want to add Purchase Order, which would be very similar. I would like to hear from developers who would be able to do this for me. Thanks!!