New to all this myself but from what I have seen & read so far 8mm would be way to small especially if you are looking for speed during cutting. Threaded bar is probably not a very good option as even if you increase the diameter you are still going to struggle to get much more than a 2mm pitch which is still going to be slow.
Although I understand you are looking to build a budget machine if you want to use it for business use then there will be a limit on how cheaply you can achieve what you need because the machine is likely to get more use that a DIY machine plus it will need a certain degree of reliability so your not constantly fixing it (takes time) & your not letting customers down. It's really an investment in your business so although you may need to watch costs don't try & do it to cheaply as it will end up costing you more in the long run.