upgrade cpu, gpu, ram, ssd?

Hi, i have an Imac 21.5 / mid 2011 / 2,5 GHz Intel Core i5 / AMD Radeon HD 6750M 512 MB and i am planning to upgrade it. I want to put an ssd Samsung 860 EVO 500GB behind the optical drive, change the cpu with the I7-2600S, the gpu with AMD Radeon HD 6770M and upgrade the ram at 16gb. The first question about the ssd is if i need a thermal sensor although i am going to connect it to an other sata port in the board and keep the hdd in place. The second question is if i am going to have problems of overheating of the gpu because the ssd behind the optical drive could possibly block the air flow, by the fan below, to the gpu. The last question is if i am going to have software (need drivers) or hardware (need a different power supply) issues although i am upgrading the imac with parts that were being put in the same model that age.