Which package to choose to build copy trading service

To make a trade copy you need usual Client(non manager) API, binaries or sources license is up to you, functionality is the same.
We have:
.NET API - $3000 binaries, $6000 sources
Java API - $4000 binaries, $8000 sources
Restful API - $4000 binaries, $8000 sources

RESTful API has much better scalability than other variants.

MT5 and MT4 have different protocols and we support them as separate products so you need to decide are you going to work with both of them or just need one of them.