Hardware Requirements (specs) for Bioinformatics-dedicated desktop
The RAM requirements for some bioinformatics analyses like assembly can be quite high (in the hundreds of Gigabytes). My recommendation is to get a fast laptop. Something with an i7 quad-core processor, 16 GB of RAM, and 1TB of storage should do. Do your analyses on this laptop. If you run out of RAM or storage, then use a cloud service to get access to bigger machines or buy an external hard drive. Once you have a feeling for what your actual needs are, start looking at a dedicated rack server solution in your local institution's server farm or again, in the cloud. 

You don't mention any analyses that clearly benefit from a GPU, but as you get more familiar with your analyses, perhaps that will become useful as well.