You're talking about big biomolecules, which usually means proteins or DNA/RNA, but gaussian (nor any QM software) is not really a suitable tool do deal with such large systems. Could be, please, more specific on what kind of chemical/biochemical systems are you interested in and what properties you want to calculate?

I am interested to biomolecules like hormones, amino acids..etc and study the interacting environment with certain molecules..I want to find the molecular geometry and vibrational frequencies of these molecules..

OK, as long as you're interested in small-molecule hormones (non-protein, amine or steroid) QM is a good option.

In short: DFT / HF / MP2 calculations in gaussian (or any common other QM software) scale pretty bad with the number of cores, so it's most efficient to run a single job on 2/4/8 cores (16 is still OK). That means that you don't need more cores for a single job, but if you have more cores in one machine you can obviously run 2 or more jobs at the same time, which also saves time.

Processor: the faster the clock-rate, the faster the calculations and (from my experience) it scales more less linearly. Buy as fast processor as you can afford. There is a great list at https://www.cpubenchmark.net/high_end_cpus.html with most of the available processors and with prices (since this is probably around 50% of the price of the system or more). If you can affor it, I would go for E5-Xeon v3 or v2 series. The fastest i7's are also commonly used for calculations, but they have limited capabilities as a good platform for calculations.

RAM memory: a single DFT job with frequencies for approx. 100 atoms should run fine with 2-4 Gb of memory. If you're running more than job on the same machine, plan accordingly and remember, that you should not use all memory for calculations. 

Disk space: .rwf files can take a huge amount of space (particularly for MP2 freq, but also for DFT freq), and hard disks are kind of cheap right now, so it's better to buy more than less. 2TB is probably the minimum, 4TB should be sufficient for some time.

Thanks for your valuable comment..