Brute force newbie questions

[rodrigo.Brasil]

1/
i am aware that the gpu can handle more faster (just wanted to simplify the example)

The program does it altomaticaly. But CPU adds only 5 to 10% more speed then my GPU. The number of CPU cores is not soo important. The program will not divide the process, as far I know, each core will get "a seed" to hash. The more core, the more seeds is tested at same time. 

But you don't need use your own computer. I like to rent server in the https://vast.ai Read the vast.ai documentation to know how to do it! 

2/
Does hashcat allow you to save ur progress on random bruteforce attacks?(in case of a power cut)

Yes, just use --restore and --session. Read the faq. 

3/
Can you give me a rough idea howlong in would take to try every combination of a 25 long string (without special characters) with my current hardware?(my new xeon server is still in transport)

Basicaly... The end of universe! With best hardware, it maybe crack it before sun goes supernova and destroy Earth! You really need to read more about it