Geist is a great sampler - primarily a drum machine but you can use it for almost any sampling task (provided it fits onto Geist's 16 pads, each of which holds 8 layers of samples). You can edit samples, apply envelopes to them, pitch and time shift etc. It comes with a full set of internal effects which you can apply to pads, layers or the entire "engine" (of which there are 8). One of the effects is a bit crusher which is great for the crunchy lo-fi sound you're looking for. You can also program patterns within Geist and indeed you can sequence your entire song in its song editor.
If you're looking to create complex instruments then maybe Kontakt would be the better choice, but for sheer usability and convenience, few programs come close to Geist. It really is an amazing piece of kit.