When I'm not using the machine, it could be used for mining?
If you plan to part-time mining, maybe you need to read up more info about pool payment systems such as PPS, PPLNS, DGM, etc. It will give you information about a correlation between time stay and miner's rewards on the pool. Some pool payment scheme even can be tricked by the practice of mining called Pool-Hooping (popular 1-2 years ago).
And would I be able to get a 100% return on investment before the end of 2018?
Depend on many things.
The other option would be to get an AntMiner instead of multiple graphic cards. So, what are your thoughts?
A few threads have given a comparison of ASIC and GPUs, may, can be your reference. Use the search function.